Molecular Oxidation
High voltage electrical discharge provides the necessary energy to increase the surface tension of low energy substrates like polyethylene and polypropylene. This surface energy corona treatment modifies the chemical composition of the material surface by introducing polar functional groups such as carbonyl and hydroxyl radicals. These added functional groups improve the wetting characteristics of the substrate during the application of inks, coatings, and adhesives.
The modification stops once the density of polar groups reaches a plateau determined by the power density of the discharge and the specific polymer chemistry of the film.
Adhesion Mechanics
Effective bond formation relies on the creation of a high energy interface between the substrate and the applied liquid material. Poor wetting occurs when the surface energy of the plastic remains lower than the surface tension of the printing ink or adhesive, which results in droplets rather than a uniform film. Surface energy corona treatment shifts the contact angle of the applied liquid toward zero, allowing for complete coverage and structural anchorage.
Printers adjust the intensity of this treatment based on the dyne level requirements of the specific ink or laminating adhesive in use. Variations in the treatment level correlate directly with the bond strength observed in finished packaging, provided that the material has not suffered thermal degradation from excessive exposure to the electrical arc.
Operational Parameters
Process control requires monitoring the web speed, electrode gap, and power input to maintain a consistent treatment level across the entire roll width. Insufficient exposure yields inconsistent ink adhesion that fails during subsequent conversion steps like heat sealing or pouch formation. High power settings create pinholes or induce back-side treatment, which causes the material to block or stick to itself when wound into a tight master roll.
Technicians periodically verify the treatment intensity using standard dyne solutions that measure the surface wetting threshold in mN/m. Uniform discharge distribution prevents localized under-treatment that threatens the structural integrity of the final composite structure.
