Pre-treatment and activation of surfaces
The set-up of a corona discharge system generally consists of one or several high-voltage electrodes arranged at a distance of between 1 to max. 12 mm from a grounded roller. If a high voltage is applied to the electrodes, a corona discharge occurs in the gap between the electrodes and the roller, and a plasma is formed. The electrodes and/or the roller are covered with a non-conductive barrier (e.g. ceramic, quartz or silicone), which has also given rise to the term barrier discharge. Compared with metal electrodes, barrier electrodes result in more even surface treatment and are suitable for treating conductive materials such as aluminum foils.
If a plastic is guided over the grounded roller through the discharge gap, its surface becomes oxidized when this is done. The oxygen in the air serves as the raw material for this oxidation. This reaction is initiated by high-energy electrons in the discharge gap, which cause bond dissociation on the plastic surface.
As a result of the oxidation reactions, polar groups containing oxygen are implemented in the surface, resulting in improved wettability of the surfaces and in most cases also improved bonding on the plastic surface.
As the amount of energy introduced into the running web increases, we find that wettability increases also. The corona dosage is often used as a parameter for the energy input:
Corona dosage = generator power / (web speed * treatment width)
e. g. = 2000 [W] / ( 50 [m/min] * 2,0 [m] ) = 20 [W * min / m²]
As the corona dosage increases, so too does the number of polar groups and thus the wettability. One measure of wettability is the surface tension, which on smooth surfaces such as foils can be reliably and easily determined with test inks.
In practice, surface tensions of 38 to 48 mN/M are required for plastics finishing, depending on the application. The corona dosage required for this is material-specific and depends on the type of plastic, the additives and the crystallinity. In many cases, a prior test on a material sample is therefore appropriate. This can be carried out in the Ahlbrandt laboratories.
- Blown - Film Extrusion
- Cast - Film Extrusion
- Label Printing / Narrow Web
- Panel Material
- Molded Parts / Profiles
- Addition Of Gases And/Or Aerosols
- Cable Treatment
- Special Materials
- Ozone Converter
- Test Pens and Test Inks