Part 2: How do you achieve good treatment with the corona process?

With the corona process – the correct term would be with a dielectric barrier discharge with air as the process gas – the hydrogen atoms on the surface of the polymer are partially replaced by oxygen atoms. As the oxygen can be taken from the air, the corona process does not require any additional gas supply or infrastructure. This makes this process very „elegant“ because it is relatively simple – no additional gas supply or coating application – and inexpensive. This fact also explains the very widespread use of this process in daily practice.

Due to the higher electronegativity of oxygen (3.44), charge differences occur on the surface of the polymer. The surface becomes more polar, and the surface tension is increased.
However, an important second effect of corona treatment is chain scission. This is caused by the fact that not only oxygen molecules (O2) occur during corona treatment, but also atomic oxygen (O). This atomic oxygen is very reactive.

This very high reactivity is also the cause of the ozone (O3) typical in the corona process and is primarily responsible for the breaking up of the polymer structure, the so-called chain scission.

This is often only seen as a negative side effect, but that is not entirely correct. This is about an optimum, i.e. maximization under a secondary constraint. The chain scission initially improves the entanglement of the polymer molecules. This results in more loose ends that can entangle, intertwine, or weave with each other.

You might have guessed it, but too much chain scission leads to polymer molecules that are too short. Much shorter oligomers or even partial monomers are formed from the polymers and such short molecules can no longer become entangled with others or with themselves and adhesion is lost. Think about the difference between spaghetti and rigatoni.

Fig. 1 Spaghetti versus rigatoni: chain scission – more is not always better

Unfortunately, these two effects, polarization and chain scission, do not exist separately.

This in turn limits the possibilities of corona treatment.

With the right amount of chain scission, the surface tension is still too low and on the other hand for enough surface tension you already have too much chain scission.
For a high number of sufficiently long chains, there is not enough surface functionalization with a short HSP distance (or at least with a similar surface tension).
Furthermore, this functionalization is far too unspecific. It always causes an increase in polarity δP and an improvement in the hydrogen bonds δH – see previous article. However, the polarity can be increased too much while the hydrogen bonds increased too little at the same time. In this way, the HSP is not close: the polymers are still not compatible with each other. As the saying goes, a miss is a good as a mile…

The third aspect of good adhesion is the crystallinity or the amorphous sections or parts within the polymer. It also only works for the amorphous sections within the polymers. Just to clarify again: The breaking of the order or crystallinity, the amorphization, can be done on the one hand by chain scission (from spaghetti to rigatoni), but also by temperature (overcooked spaghetti versus al dente). In the corona process, amorphization occurs solely through chain scission!

There is no temperature effect. No melting, no cooking. Among other effects, is also reflected in the low stability of this form of surface treatment over time.
It can be seen in the flattening of the so-called treatment curve and in the drop in the treatment effect within around three weeks, see Figure 2, the standard chart in our industry.

Fig. 2 Typical treatment curves of polypropylene using the corona process: more is not an option

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

The next article explores this question.