Plasma, corona ? Is there a difference ?
What is the difference between plasma and corona ? What are their benefits and drawbacks ?
A lot of people ask us this question. We will try to give a precise and detailed answer.
It’s all about vocabulary
There is a big confusion, because the words « plasma » and « corona » do not have the same meaning, whether they are in the physical or technological context.
Plasma in the physical context
In its physical definition, plasma is an ionized gas. It means part of its atoms or molecules have a non neutral electrical charge, either after having lost one or several electrons (which become free electrons) or after having absorbed an electron.
As a consequence, plasma can transport electrical charges, which means it can conduct electricity, as opposed to a normal gas that is an electrical insulator.
A plasma holds numerous species : free electrons, positive or negative ions, radicals, metastables. All these species and their associated electromagnetic rays can be used in the technological use of plasma.
Corona in the physical context : corona effect, lightning rods and Elmo’s fires
A popular method for creating a plasma is to apply a voltage between two conductive electrodes that are separated by a gas.
When the voltage is high enough, the electrical field accelerates natural free electrons which will then extract other electrons from the atoms or molecules they hit.
When the electrodes are higly asymetric, and especially when one of them has a needle shape, the electrical field is not uniform inside the inter-electrode space.
The electrical field will be higher on the needle-shaped electrode side.
This leads to a special situation where the plasma is created around the needle-shaped electrode, but not beyond. As a result, small arcs are cracking around the needle-shaped electrode. This is the corona effect.
This property can be be observed through a phenomenon known by sailers, who call it « Elmo’s fires ». In stormy weather conditions, a sail-boat can have micro arcs cracking at the tip of its masts, the masts being the needle shaped electrode and the other electrode being the electrically charged stormy cloud above it.
The same phenomenon can be observed on the tip of alpinists ice axes, still during stormy conditions.
The needle-shaped electrode property is used in ligthing rods, which needle-shape triggers thunder bolts more easily.
Corona in the technological context : corona treatment
Corona treatment is a technology which most popular setting is used for treating wide plastic films (several meters) at high speed (several hundreds of meters per minute). The plastic film is hold by a roll electrically connected to the ground. An electrode faces the roll, a thin gas layer (usually air) separating it from the film. The electrode is connected to a generator that delivers a high voltage signal (several kV) at several kHz. When the voltage is applied, an electrical discharge is created in the air layer. You will have probably noticed that the corona in the technological context is a plasma in the physical context ! Here begins the confusion.
The electrode profile usually has a one or several needles shapes, in order to make the electrical discharge easier, like in the corona effect. For this reason, the name « corona treatment » was chosen for the technology. This is, however, the only reason, because everything else is different ! Indeed, in corona effect (physical context), the propagation of the arcs is naturally limited by the electric field decrease, away from the needle-shaped electrode. In corona treatment, arcs are created in the whole inter-electrode space. Their intensity is limited either by the treated film itself (we talk about dielectric barrier discharge), or by the generator’s current regulation.
Some other corona equipment setups create a discharge between two electrodes and blow the resulting discharge towards the treated area. This is called blown corona.
Plasma in the technological context
Low temperature atmospheric pressure plasma is a technology based on the principle of plasma, as a physical phenomenon. It is an electrical discharge created within a gas, at atmospheric pressure, with a low current, which resulting gas is blown onto the treated area. The most frequently used gases are : air, nitrogen, helium, argon, and sometimes a mix of these gases.
The current intensity is typically 1 Amp. Therefore, even if the temperature inside the plasma can reach several hundreds of Celsius degrees, it is classified as a cold plasma, as opposed to the thermal plasma used for cutting or spraying, which can reach several thousands of Celsius degrees.
The species generated during the discharge are blown outside the discharge area, the gas flowing continuously, in order to reach the treated area. The creation of the reactive species (the electrical discharge) is clearly separated from the surface reactions (the treatment). We talk about blown plasma or remote plasma. Contrary to blown corona, the blown plasma is very homogeneous and looks like a flame. One should not see the electrical arcs outside the source.
Plasma Vs Corona : pros and cons
After having existed long before atmospheric plasma, corona treatment is now considered as the « poor man’s plasma ». Indeed, corona is an affordable technology, which makes it attractive. It has many limitations however :
- uneven treatment
- treatment that can damage the surface (craters or stains)
- pinhole effect or parasite backside treatment on films
- open air high voltage that can generate electromagnetic radiations
- conductive materials treatment is more difficult (it requires special electrodes)
- low treatment efficiency
Thanks to a clean separation between the electrical discharge and the surface treatment, plasma get rids of all the aforementioned and brings a high added value solution. Therefore, corona treatment is being more and more replaced by plasma in the industry. However, corona treatment keeps a good advance for the treatment of medium to low added value films, requiring large treatment (several meters) at high speeds (> 100 m/min). Corona treatment is also the winner with its low cost, as long as the drawbacks are acceptable for a given application.