Good wettability is a prerequisite for the adhesion of a surface with the bonding partner in the painting, gluing, printing or bonding. Wetting is not only impaired by contamination with oil and grease; many materials have surfaces which, even if clean, cannot be sufficiently wetted by many liquids such as adhesives and paints. The liquid rolls off. It will not adhere to the surface even after curing or drying.
The reason is that the surface energy of the substrate is low. Materials with lower surface energy can wet materials with higher surface energy, but not vice versa. Therefore, the surface energy of the applied liquid, also known as surface tension in the case of liquids, must be lower than that of the substrate.
The surface energy of most plastics is too low to be wetted by adhesives and coatings. This is due to their non-polar surfaces. No attachment site is available for the applied liquids.
In plasma activation, the surface is activated by increasing surface energy. In this process, attachment sites are created for the applied liquid.
Conventional activation uses chemical primers as liquid adhesion promoters, which are highly corrosive and harmful to the environment. On the one hand, they require adequate evaporation time before further processing; on the other hand, their activated state cannot remain too long. Also, non-polar materials such as polyolefins cannot be sufficiently activated by chemical primers
In addition, activation can be carried out in arch or corona discharge. This is a form of atmospheric pressure plasma treatment. However, it can only treat flat or convex surfaces, which are introduced into the arc during such treatment.The arced plasma is blown out through a nozzle. This allows the activation of component surfaces with complex curvature.
During activation in an air or oxygen plasma, the non-polar hydrogen bonds of plastic polymers are replaced by oxygen bonds which provides free valence electrons for bonding with liquid molecules.
Plasma activation at low or atmospheric pressure provides the surfaces of “non-glueable” plastics such as POM, PE and PP with excellent adhesiveness and sprayability. It offers accurate adjustment options for the desired surface energy; this can prevent over-activation, which can result in undesired surface etching.
In low-pressure plasma, other gases besides air and oxygen can also be used, which can, in place of oxygen, accumulate nitrogen (N2), amines (NHx) or carboxyl groups (-COOH) as reactive groups.
The activated state of plastics can last from several weeks to several months. However, it is advisable to carry out follow-up processing as soon as possible, as a cleaned surface will still attract dust, organic contamination and while in storage.
Plasma treatment can also give PTFE adhesiveness. However, this is not achieved through activation but by etching.
Generally, the surface energy of metals, ceramics and glass is higher than that of plastics. Nevertheless, there are also applications where these materials benefit from plasma activation. The surface tension of solder alloys is likewise high, so they roll off many metal surfaces. Therefore, plasma activation of metals can be used to improve wetting during soldering. It should be noted that the activation of metals is mostly effective only in a few minutes, and (inline) soldering must be carried out immediately after activation.