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Why is ink the better finger?
Why does glue or adhesive tape stick? How do we test tack? Everything about the finger test, ink test, and the surface energy in gluing.
When you critically check adhesive tape by finger, you may think that its tack is high if the tape sticks to your finger. But that does not mean that the tack is the same on other materials. The reason why something glues depends on the surface energy. What does that mean for your finger? Find out here.
How does glue/adhesive tape work?
How strong an adhesive bond should be depends on the application. If the adhesive tape should be easy to remove, as is the case with masking tape for example, without damaging the surface, it should not be too strong. If it needs to hold things together for years, such as the company logo on the back of your car, it must be so strong that it cannot unstick under any circumstances or conditions. The bonding properties are specified very precisely for each adhesive tape. Many factors play a role here: The adhesive, the backing and the application weight are a few of these.
The strength of the adhesive tape’s bond depends on very different things: the adhesion conditions (temperature, etc.), the application (e.g. application pressure), the heating time and pull-off speed – and also the surface to be bonded. The polarity and characteristics (raw/smooth) of a surface play an important role in the adhesion property.
Surface energy: bonding with tension
Whether or not adhesive tape adheres well to a surface depends primarily on the surface energy. Only what’s “right” in many different contexts can bond in an ideal way.
Let us stick with the tailgate of your car for the time being. Like every Saturday, you have just washed and waxed it. And naturally it begins to rain on it. However, the drops of water simply run off. They keep their form and do not dissolve.
The reason for this is that they have a higher surface energy than the waxed car paint (in a Teflon pan the drops would behave the same). This is because a liquid fundamentally does not want to dissolve. It tries to keep its surface as small as possible. To this end, it forms a ball on account of its surface tension.
However, some forces act on such a ball. Gravity attracts it and also other surfaces such as your car paint. The stronger the force attracting the ball, the more it becomes deformed. The ball dissolves on the surface if there is a strong interaction with a high-energy surface.
It is similar with the adhesive of adhesive tape and the surface on which you would like to attach the tape. Independently of the surface (and its individual surface energy), the adhesive will dissolve better in one case, worse in another. It glues more solidly in one case and less solidly in another. That is why primers are used for adhesive tape with backings that have low surface energy (for example, plastic). They increase the surface energy of the backing so that the adhesive adheres even more strongly.
Finger or ink?
You can stick your finger in again if you want to test the tack of adhesive tape. But the extent to which adhesive tape sticks to your finger does not say anything about the tack on the surface that you want to use it for. Your finger and a surface have quite different surface energy (unless you want to stick the adhesive tape to your finger, of course. Then you can also do the test with a plaster...).
Why is ink the better finger now? It's quite simple: because ink is liquid and acts like an adhesive to a certain extent. The more evenly the ink dissolves on the surface, the higher the surface energy is. This shows what properties the adhesive must have in order to stick to this surface. You will find out about how this works in our article on the ink test.