Ink Test

How Can You See the Future with an Ink Ball?

Test methods & research

The surface tension determines how firmly adhesive tape sticks to the surface. The “ink test” shows it.

Why does the same adhesive tape not stick equally strongly to different surfaces? It’s all a question of surface energy and surface tension! The ink test and the necessary information will help you figure out how well the adhesive will cling and glue to a surface.

Surface tension of water molecules
Surface tension of water molecules

Surfaces: Tension and Energy

Surface tension and surface energy are two different things. You can picture it very easily: In a liquid, the molecules move in every direction; the intermolecular forces entirely neutralize each other. This is different on a surface: here, the intermolecular forces act from above; the molecules move to the inside, into the liquid.

It forms a “skin” on the outside, which is held together in the air by the surface tension. Surface tension is the reason that liquids want to keep their surface as small as possible. That is why ink drops can assume the perfect form of a ball. The surface energy, by contrast, is the amount of energy that is needed in order to break the bonds so that a new surface results. Both terms are often used as synonyms for liquids.

On low energy surfaces liquids stay in ball form. On high energy surfaces liquids run evenly, which means: this is a good place to tape on.
On low energy surfaces liquids stay in ball form. On high energy surfaces liquids run evenly, which means: this is a good place to tape on.

High Energy, Solid Bond

The surface energy of a material decides how well adhesive tape can stick to it. The following applies as a matter of principle: The higher the surface energy of a material, the better the adhesive tape or the adhesive will stick to it. That is because high surface energy ensures that the adhesive can form a new surface quickly and flow broadly over the surface. A simple “trick” for measuring the surface energy is the ink test. This is as simple as it is conclusive: On the surfaces with high surface energy, the ink runs evenly; the adhesive will therefore stick to them easily. In the case of surfaces with low surface energy, the ink drops will remain a ball and run off. Adhesive does not stick as well to these surfaces.

A Peak in the Ink Ball

This is how you can see into the future with an ink ball: Measuring the surface tension is helpful if you are not entirely sure what the properties of the surface to be bonded are. The surface tension ink test is a reliable tool in such situations since this test can be done quickly and easily. The best thing is: at home you can also measure the surface tension without difficulty. You just need to apply drops of ink to a surface and watch how the drops form or dissolve. This is because the adhesive would also act in a similar way: either flowing broadly over the surface or not.

Where Is it Best to adhere?

Low energy surfaces such as plastic are hard to stick to – ink drops will run off the surface. The same applies to adhesive. Low-energy surfaces such as synthetics are a special challenge, for example. If you choose a less suitable adhesive tape, it will peel off more easily. Difficult surfaces with poor adhesive properties are considered to be polyethylene (PE), polystyrene (PS), polytetrafluorethylene (PTFE), polypropylene (PP), silicone or powder coatings.

By contrast, there are materials with high-energy surfaces. Adhesive sticks to them very well. The adhesive spreads widely and evenly on the surface. High-energy surfaces include steel, aluminum, polyvinyl chloride (PVC) and polycarbonate (PC). Adhesive sticks to them very easily and reliably.

But there are also solutions for everyone who would like to attach adhesive to surfaces with low surface tension. The result of the measurement is not critical for the resulting adhesive bond. Surface tension can be changed very easily with primer. It increases the surface energy chemically and ensures that the adhesive will be able to stick well. Incidentally: thorough cleaning and degreasing of a surface often produces miracles. The surface energy also changes here.

Not Well-rounded? Great!

At its core, the surface tension ink test tells you one thing above all: If the adhesive does not want to become microscopically tiny balls as quickly as possible, if it disperses well, and if it is not “all round” on the adhesive tape, then you have found the perfect partner for the surface to be glued.