PROCESS: HOW CORONA DISCHARGE WORKS

When it is necessary to bond plastic materials to metals or other plastics, or to simply print on a plastic surface, quality results depend on the ability of the liquid, adhesive or ink to adhere to the surface and this is where corona discharge comes into place. Adhesive strength relies on one specific property of the surface: the surface tension.

Surface tension energy is measured in dyne/cm (mili N/m). Surface tension energy is the deciding factor on how well a liquid adheres to a polymer surface. For a proper bond to exist between a liquid and a substrate surface, the substrate's surface tension energy must exceed the liquid's surface tension energy by about 2-10 dyne/cm. The higher the surface tension energy of the solid substrate in relation to the liquid, the better its 'wettability'. For information regarding necessary dyne levels and surface tension energy, please see the chart below.

Corona Discharge Process	Corona Discharge Process	Corona Discharge Process

 

Many engineering plastic materials have insufficient surface tension energy for printing or bonding. Polypropylene and polyethylene are prime examples. These materials have many useful properties, which make them materials of choice. However, their poor wettability creates severe limitations when it comes to design choices.

What Does Corona Discharge Do?

3DT's Corona Treating Systems radically improve the wettability, surface tension energy, of the surface of these materials through the use of a corona discharge. It provides a surface treatment that enhances the ability to bond or print on virtually any three-dimensional polymer surface. The corona discharge increases the surface tension energy of the material to strengthen its wettability. This creates a powerful bond between the liquid and the material surface for maximum adhesion.

When atmospheric air is exposed to different voltage potentials, electrical discharge can develop. When this occurs, it results 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 the Corona discharge - which is then used for surface treatment of plastic substances.

When a plastic substance is placed under the corona discharge, the electrons generated in the corona discharge impact on the treatment surface with energies two to three times that necessary to break the molecular bonds on the surface of most substrates. The resulting free radicals react rapidly with the oxidating products of the corona discharge, or with adjoining free radicals on the same or different chain, resulting in a cross-link. Oxidation of the solid surface increases the surface tension 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.

Necessary dyne levels (surface tension energy) for corona discharge:

Process	Pad/Screen Printing			Offset Printing			Coating/Laminating
Material \ Ink	Solvent	Water	UV	Solvent	Water	UV	Solvent	Water	UV
LDPE	38-44	42-48	46-60	38-40	38-44	40-50	38-45	42-50	48-56
HDPE	38-44	42-48	46-56	38-40	38-44	38-50	38-44	42-50	48-56
PET	38-44	48-60	44-62	40-46	38-44	38-50	42-48	42-60	48-56
PP	38-44	42-48	44-60	38-40	38-44	40-50	38-44	42-50	48-56
PVC	38-44	42-48	42-60	38-40	38-44	38-44	38-45	40-48	48-56
EPDM	N/A	N/A	N/A	N/A	N/A	N/A	42-50	44-56	48-56
EVA	38-44	42-48	42-60	38-40	38-44	38-50	38-44	42-50	48-56