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Corona treatment, plasma, ozone and NOx

Corona treatment is often used to improve the adhesion properties of plastics.
The most common plastics have a non-polar, water-repellent and electrically insulating surface. Such surfaces do not readily allow themselves to be wetted by inks, adhesives or adhesion promoters. As a result, the plastics are often difficult to bond or print on. For example, PP, PE or PS films cannot be printed, laminated or coated without corona treatment. Corona treatment allows the polarity (or surface energy, surface tension) to be increased, resulting in improved wettability. In most cases, this also results in improved adhesion. Surface tension is given in mN/m or also referred to as dyn value.

In foil production, corona treatment is often integrated in the extrusion line. This allows the foils to be processed immediately. The foil is affected by a high-voltage electrical discharge (also called plasma). The high-voltage discharge occurs between a grounded roll made of steel or aluminum and an often ceramic-insulated electrode. Depending on the process, the use of an insulated roller against an uninsulated electrode is also possible. There must always be insulation (dielectric) between the electrodes to prevent a direct short circuit. The material to be treated (substrate) can also serve as insulation. Corona systems are often integrated into printing presses alongside UV lamps. UV lamps also generate ozone, and therefore print shops often require ozone extraction.

With insulated electrodes, even conductive (and grounded) substrates can be treated. Corona (or plasma) discharge is usually generated by means of high voltage generators (10-40 KV) with frequency ranges between 10 and 60 kHz.
Spatial shapes can also be corona treated. Examples are disposable syringes, cables and plastic containers.

The corona discharge leads to ionization of the air and ozone (3 oxygen molecules, O3) is formed. Ozone is a colorless to slightly blue and chlorine-like smelling gas. The odor threshold for human perception of the gas is about 40 µg/m³. Ozone is a strong and toxic oxidant and can therefore cause irritation of the respiratory tract and eyes as well as respiratory diseases. Furthermore, there is a well-founded suspicion that O₃ has a carcinogenic effect on humans.
Ozone is an unstable molecule that decomposes to oxygen in a short time. In addition, O₃ is very reactive (oxidizing) and has a corrosive effect in the process. Ozone promotes fire, can form ignitable and explosive mixtures and is heavier than air.
Activated carbon filters can convert ozone into oxygen (O2). This means that discharge to the outside is not necessary, but in some cases it is advisable.

Certain corona technologies and all atmospheric plasma technologies generate significant emissions of nitrous gases (nitrogen oxides). Nitrogen oxides (NOx) are a combination of nitrogen monoxide (NO), nitrogen dioxide (NO2) and other nitrous gases. Nitrogen oxides are harmful to health and cause lung diseases, among other things.

Nitrogen monoxide (NO) is a colorless, non-flammable gas. It reacts with oxygen and ozone (O3) to form nitrogen dioxide. Pure nitrogen dioxide (NO2) is highly oxidizing, very toxic and harmful to both organic and inorganic materials. NOx and ozone can thus attack machine parts and cause rust damage, for example. To avoid such damage, precise and, if possible, near-source extraction is required.

According to the Federal Environment Agency, NO2 as a strong oxidant leads to inflammatory reactions in the respiratory tract. The consequential effects are shortness of breath, cough, bronchitis, pulmonary edema, increased susceptibility to respiratory infections, and reduced lung function. The indirect effect of NO on human health is its capacity as a precursor for particulate matter. NOx is barely odorless in harmful concentrations. NOx can only be eliminated at very high temperatures via filter processes and therefore for most manufacturing processes and companies, discharge to the outside is the only solution.