Plasma technology: a technology for surface cleaning ?
Are plasma technologies, which are reputed to be clean as they produce very little waste, suitable for surface cleaning? Can they be considered as solutions to degreasing problems?
To answer these questions, we need to understand the physico-chemical reactions that can be used to remove surface pollution. To achieve this objective, the following two conditions must be met :
On the one hand, the contamination molecules and the gas molecules from the plasma must react together.
On the other hand, the reaction products must be sufficiently volatile to be eliminated by the renewal of the atmosphere above the treated surface.
The overall efficiency of this process is generally enhanced by factors such as temperature, the presence of UV radiation or ion bombardment.
Post-discharge’ plasma technologies, where the plasma creation site is different from the area of use, are potentially better suited to the treatment of shaped products. They are generally more immune to surface contamination, so treatment performance will remain much more stable over the long term.
Surface cleanliness: a difficult concept to describe simply
It’s not easy to characterise a surface cleaning process because the concept of surface cleanliness is difficult to define. The most effective tools for this purpose are very expensive (XPS surface analysis, for example) and incompatible with production rates. There are simpler methods, but they can lead to a poor assessment of the result.
Great care must be taken when using test inks to qualify surface cleaning. The initial contamination, which is generally hydrophobic, may only be partially removed by the plasma treatment. The remainder being transformed to become more hydrophilic; the surface could be wrongly perceived as clean.
Types and quantity of contamination
Contamination is characterised by its nature and quantity per unit area.
Most often it is organic matter or long molecular chain hydrocarbons. They sometimes contain mineral substances. It is also possible to encounter evanescent and water-soluble products, particularly on aluminium products.
Quantities can vary greatly from one situation to another, from a few mg/m2 to several hundred mg/m2. For some products, quantities can also vary greatly from one product to another.
Effectiveness of a plasma treatment
Experimentally, it has been observed that the effectiveness of plasma treatment decreases sharply with the lengthening of the molecular chains of the pollutants and the quantity to be removed.
When the contaminants are present in the form of long chains or in large quantities, chemical reactions transforming the pollutants (cross-linking, polymerisation, etc.) have been observed under the action of plasma treatment, reducing the effectiveness of the cleaning.
Similarly, in the presence of mineral pollution (fingerprints, stearates, etc.), surface cleaning results are not conclusive.
However, in the presence of a small quantity of short-chain contamination, it is possible to obtain a very effective cleaning process. This situation generally arises when other cleaning methods have been used prior to plasma treatment, which then provides a finishing clean.