Corona is generated by applying a high frequency, high voltage signal to an electrode separated from an earthed plane by an air gap, the substrate and a layer of dielectric material.
Frequencies in the range of 9 to 50kHz with peak voltages up to 30KV are generally used in the corona treatment industry. When a voltage is applied to the electrode, virtually no current will pass through the air gap until the electrical breakdown of the air takes place. This occurs when the voltage across the air gap reaches a level of 3,000-5,000 volts per millimetre, the variance being caused by ambient conditions such as humidity.
At breakdown potential, free electrons are accelerated towards the positive electrode with such high energy that they are capable of displacing electrons from molecules in the air gap. The consequence is an avalanche effect, with electrons and the corresponding ions being produced, resulting in current flow across the air gap. This breakdown of the air produces large ionisation currents and as the input power to the corona is increased, the potential across the air gap remains essentially constant.
In a corona treatment system, as the voltage on to the discharge electrode is an a.c. (alternating current) voltage, the electrodes are alternatively positive and negative with respect to each other. The electrons and positive ions are therefore constantly changing direction. As the ionisation current increases, the power in the corona discharge increases proportionally, i.e. the rate at which the charged particles are moving, increases.
For a given power, producing a relatively high rate of electron bombardment on the surface of the film, a small air gap (and its resulting lower breakdown voltage), will produce a relatively low energy level. This is the result of a lower breakdown voltage and smaller gap restricting the acceleration of the particles. Increasing the gap increases the breakdown voltage and correspondingly the energy level of each particle is increased. However, the rate of bombardment of the particles on the film is reduced, therefore, simply increasing the breakdown voltage and the air gap does not increase the corona power onto the substrate. Many other factors play an important role in a correct and effective corona discharge, such as corona power supply, frequency of operation, air gap settings, dielectric materials and discharge electrode configurations.
This is why it is of utmost importance to have you corona treater specifies not only correctly and bespoke, but to have equipment which takes these factors into consideration.
Consult with Corona Supplies
technical experts for further information