The Corona Process
Let’s take a look at how corona surface treatment works. 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 is impacted by electrons with the energy two to three times that necessary to break the molecular bonds on the surface. This leads to the formation of free radicals, which rapidly react with the oxidation of products in the corona discharge or with adjoining free radicals on the same or different chain, resulting in cross-links. 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 the development of strong oxidants is not essential for adhesion to take place, wetting tension and improved surface energy is most assuredly related to the oxidation of a polymer surface by the creation of polar groups, such as hydroxyl, carbonyl and amide groups.
Corona treatment effectively improves the surface tension on numerous materials that show poor or no adhesion properties to media such as printing inks and adhesives. The corona treatment of plastic and other materials ensures that virtually any bonding problem can be successfully solved, even for tough-to-bond polymeric and elastomeric materials.
Several types of tools are used to measure the surface energy or wettability of a material, both before and after treatment. Frequently dyne pens and dyne ink are used to assess the dyne level, as demonstrated in the video on the left. Learn more about dyne level and these products on the tabs to the left. Also employed is a tensiometer to measure the contact angle of a drop of water on a substrate. A low contact angle provides better adhesion. See the infographic to the left.
A newer tool used to measure surface energy is an automated surface analyzer. This system ensures quick, accurate results by calculating the contact angle at the push of a button, without the risk of human error.
In the photos below note how the water droplet has a lower contact angle on the corona treated surface. Clear proof that 3DT’s corona treatment equipment creates significant improvement in functionalizing surfaces.