The Corona Process
When atmospheric air is exposed to different voltage potentials, electrical discharge develops resulting in an avalanche effect caused by the collision of neutral molecules and the electrically loaded molecules which make up the voltage. Upon collision, the neutral molecules become electrically loaded, resulting in a heavily loaded zone or “lightening”. This, in turn, creates a heavy oxide mixture of ozone and nitrogen oxides. To avoid this avalanche effect, an isolator is placed between two electrodes. The result is a cloud of ionized air – or corona discharge – which is then used for the surface treatment of plastics and other materials.
Consequently, a substance placed under the corona discharge are impacted by the electrons with energy two to three times that necessary to break the molecular bonds on the surface. The resulting free radicals rapidly react with the oxidation of products in the corona discharge or with adjoining free radicals on the same or different chain, resulting in a cross-link. Oxidation of the surface increases the surface tension or surface energy allowing for better wetting by liquids and promoting adhesion. Though studies have shown that development of strong oxidants is not essential for adhesion to take place, wetting tension is most assuredly related to the oxidation of the polymer surface resulting in polar groups on the surface, primarily hydroxyl, carbonyl and amide groups.
Corona discharge effectively improves the surface tension on numerous materials that show poor or no adhesion properties to media such as printing inks and adhesives. By understanding the basic science of corona surface treatment, virtually any bonding problem can successfully solved, even for tough-to-bond polymeric and elastomeric materials.
Measuring the effectiveness of surface treatment frequently employs a tensiometer to measure the contact angle of a solution (typically distilled water) on a substrate. A Dyne Solution Test Kit is another useful way to evaluate surface wettability as seen in the demonstration to the left.
In the photos below note how the water droplet has a lower contact angle on the corona treated surface. Clear proof that corona treatment creates significant improvement in functionalizing surfaces.