Part 3: Are there other plasma processes that offer a better balance between the three key aspects for good adhesion

Yes, there are other atmospheric pressure plasma processes that can be found in practice. However, these are either very expensive or they are even less well balanced for the treatment of polymer films than the corona process.

1. Very expensive, as they require complex installation, such as an additional gas supply for flame treatment or the CAPS treatment technology (Controlled Atmosphere Plasma System – see more on our website).
2. Less well balanced. Plasma processes such as plasma jets or EDGE treatment technology should be mentioned here. They use far too much energy and thus cause too high a temperature load for films. However, the treatment of polymer films is neither the intended application of these two. On the other hand, they are very well suited for the treatment of voluminous materials. The jets are better for molded parts, EDGE (Enhanced Discharge Geometry) is good for foams, nonwovens or very sensitive materials. By the way, both technologies are being supplied by Plasmawerk.

FLAIR® stands for Flame In AIR.

The Flair process also uses ambient air as a process gas and causes an amorphization of the polymer and increases the surface tension, as well. However much higher values can be achieved and these in turn enable a couple positive effects, such as a significantly better storage stability of this effect for instance.

However, one process combines the „elegance“ of the corona process with a better balance of the three key aspects for good adhesion: the Flair process.

Figures 1 and 2, Corona and flair, surface tension (left), adhesion (right)

Question: Why is that so?
Answer: Thanks to the better balance of the three essential aspects.

The first aspect, chain scission: Due to a change in the way the plasma is generated, less atomic oxygen is produced in the flair process, which means that almost no ozone is produced, in contrast to the corona process. As a result, the molecular weight or length of the molecules remains above the critical level.

The other aspect: The HSP distance and surface tension. For a small HSP distance, at least the surface tensions should be close to each other. Even better if the disperse and the polar part of the surface tension are close to each other and even better if all HSPs are close to each other. A lower chain scission leads to a significant increase in surface tension. This opens up a larger range for the input of specific energy that can be addressed or targeted. It allows to move within a larger radius in the HSP parameter space.

This is very good and could be seen as the main advantage of Flair over Corona. This larger radius of action means that the treatment window, which is advantageous for good adhesion, can be extended and improved compatibility with the respective coating can be achieved.

Or to put it another way, more specific energy does not lead to more chain scission, so there is no limit as in the corona process.

There is also an additional effect with the Flair process: The amorphization of the polymer through temperature.

The physical geometry of the flair treatment means that the temperature applied to the surface of the polymer is very high at certain points. However, only the uppermost molecular layer in the nanometer range is treated. This is why the surface of the polymer cools down again instantly. To a certain extent, the crystalline molecules on the polymer surface melt and thus lose their order, become amorphous and then immediately freeze again in this amorphous state.

An additional rearrangement through melting and freezing.

To summarize

• The Flair process enables a much higher adhesion due to the very good balance of the three critical aspects.
• No ozone is produced.
• This results in a further advantage: no need for a roller coating. A durable stainless-steel roller is sufficient.
• It is also very compact, making it very easy to integrate into an existing system and – like a corona – easy to operate.

The following and final article in our series draws a comparison between corona and flair and lists some conditions under which the respective processes are suitable.