short note on corona discharge surface treatment on plastics

Surface treatment with high voltage discharge modifies only the surface characteristics without affecting material bulk properties.

Materials

Many plastics, such as polyethylene and polypropylene, have chemically inert and nonporous surfaces with low surface tensions. hence they are non-receptive to bonding with printing inks, coatings, and adhesives. Although results are invisible to the naked eye, surface treating modifies surfaces to improve adhesion.

Polyethylene, polypropylene, nylon, vinyl, PVC, PET, metalized surfaces, foils, paper, and paperboard stocks are commonly treated by this method. It is safe, economical, and delivers high line speed throughput. Corona treatment is also suitable for the treatment of injection and blow moulded parts, and is capable of treating multiple surfaces and difficult parts with a single pass.

Equipment

Corona discharge equipment consists of a high-frequency power generator, a high-voltage transformer, a stationary electrode, and a treater ground roll. Standard utility electrical power is converted into higher frequency power which is then supplied to the treater station. The treater station applies this power through ceramic or metal electrodes over an air gap onto the material’s surface.

Three basic corona treater stations are used in coating applications. Bare Roll and Covered Roll and Universal roll. On a Bare Roll treater station the dielectric encapsulates the electrode. On a Covered Roll station it encapsulates the treater base roll. The treater consists of an electrode and a base roll in both stations. In theory a Covered Roll treater is generally used to treat non-conductive webs, and a Bare Roll treater is used to treat conductive webs. However, manufacturers who treat a variety of substrates on the same production line may choose to use a Bare Roll treater.

In a covered roll station the roll is insulated with some type of dielectric material and the electrode is bare metal. In a bare roll system the roll has no insulation, but the electrode is insulated. In a universal roll station both the ground roll and the electrode are insulated.

Covered roll system can only treat non-conductive substrates. Bare roll or universal roll systems can treat both conductive and non conductive substrates.

Shelf life of treated surfaces

The shelf life of pre-treated materials ranges from hours to years, depending on the plastic, its formulation, how it was treated and its exposure to elevated temperature after treatment. Material purity is the most important factor. Shelf life is limited by the presence of low molecular weight components such as antiblock agents, mould release, antistatics, etc. Eventually, these components migrate to the surface of clean polymers. It is therefore recommended to print or bond to the material soon after treatment. However, once the treated surface has been interfaced with a coating, ink, adhesive, or another material, the bond becomes permanent.

Applications:

·         Treatment of surfaces of bio-medical testing devices to improve wettability of surfaces for confluent liquid flow.

·         Treatment of syringe barrels prior to printing.

·         Treatment of the inner surface of needle hubs prior to bonding a stainless steel needle.

·         Treatment of electronic cable insulation to improve adhesion of inks and coatings.

·         Treatment of lids and covers of chemical containers prior to gasket material application or printing.

·         Treatment of plastic bottles prior to application of adhesive labels.

Treatment of automotive profiles made of EPDM rubber prior to application of an adhesive for retaining flocking bristles or decorating fabric.