Plasma activation

A prerequisite for the adhesion of binding partners for painting, gluing, printing or bonding is a good wettability of the surface. Not only is wetting prevented by oil and grease marks, but also the clean surface of many materials cannot be sufficiently wetted by many liquids, adhesives and inks. The liquid rolls off. It will also not adhere to the surface after curing or drying.

The cause is a low surface energy of the substrate. Substances having a low surface energy wet those with a high surface energy, but not vice versa. The surface energy of the applied liquid, also referred to as surface tension, must be lower than that of the substrate in each case.

Most plastics have a very low surface energy, too low for wetting by adhesives and coatings. The reason is the non-polar surface. The molecules of the liquid cannot find connection points where they can accumulate.

The surface energy of a surface is increased by being activated. This ensures that attachment sites are created for the applied liquid.

Activation is traditionally achieved using chemical primers, liquid adhesion promoters. They are often highly corrosive and harmful to the environment. On the one hand you need to allow adequate drying time before further processing and on the other hand the surface often does not remain active for long. Non-polar materials such as polyolefins are not sufficiently activated by chemical primers.

You can also activate surfaces in an electric arc corona. This is a form of atmospheric pressure plasma treatment. However, only flat or convex surfaces that can be introduced into the electric arc can be treated in this way.

With Diener electronic atmospheric pressure plasma systems, the plasma for the electric arc is blown out through a nozzle. This also allows the surface of complex curved components to be activated.

Upon activation by air or oxygen plasma, non-polar hydrogen bonds of the plastic polymers are replaced by oxygen bonds. These can provide free valence electrons for the binding of liquid molecules.

Plasma activation under low pressure or atmospheric pressure also allows "non-adhesive" plastics such as POM, PE and PP to have very good bondability or be paintable. The desired surface energy can be adjusted very precisely, so that over activation that leads to etching can be avoided.

In low-pressure plasma, other gases can be used instead of oxygen or air, for example by use in place of oxygen, nitrogen (N 2), amines (NHx) or carboxyl groups (-COOH) are attached as reactive groups.

The activation of plastic surfaces remains effective over weeks and months. Further processing should still take place rapidly, since with increasing aging new contaminants are deposited.

PTFE can also be glued by plasma treatment. However that is not through activation, but rather due to etching.

Metals, ceramics and glasses generally have higher surface energies than plastics. However, there are also applications for these materials, in which plasma activation creates advantages. The surface tension of solder alloys is high and they roll off on many metal surfaces. Therefore, the plasma activation of metals also improves wetting during soldering. However, the activation of metals is usually only effective for a few minutes and they must be installed directly upstream of the soldering process (in-line).



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