Coronal Discharge | Corona Discharge for Surface Modification

Corona Discharge

Corona Discharge - Enhancing adhesion capabilities

In the manufacturing world, specific materials are preferred for producing specific products. While those materials may have positive characteristics such as malleability, strength, and heat resistance, for example, they may not have the surface energy to accept adhesives, coatings, paints, or printing well. If those treatments don’t adhere properly, they peel, flake, and fail. The result is a low-quality product.   

Plasma_APC_500_VideoausschnittThe low surface energy of certain materials often leads to poor adhesion of inks, glues, and coatings because they possess significantly high surface energies. To achieve optimum adhesion, it’s vital that the base material’s surface energy is equal to or higher than the surface energy of the substance which is applied to it, such as an adhesive or paint. Fortunately, a corona discharge can be used to effectively activate a surface in order to make it more receptive to a coating. 

What is a corona discharge?

A corona discharge is a surface modification technique that uses a low temperature corona discharge plasma to impart changes in the properties of a surface. Corona plasma is generated by applying high voltage to an electrode with a sharp tip, where the plasma forms. A linear array of electrodes is often used to create a glowing bluish-purple curtain of corona plasma through which material to be treated is passed. This causes surface activation of the material, increasing its surface energy by bombarding it with plasma ions.

Plasma treatments come in two basic processes: vacuum and atmospheric plasma. Vacuum plasma utilizes a vacuum pump to remove most of the air from a chamber, creating a low-pressure environment conducive to ionizing the remaining air or gases with a strong electrical field that produces the plasma.

Corona discharge is an atmospheric plasma, which requires no vacuum. Produced by electrically energizing air or gases as they pass through a nozzle at surrounding atmosphere or normal pressure using a pulsed electric arc generated by high voltage discharge, it enables plasma treatments to be applied directly in-line with a manufacturing assembly process. Without the need for a vacuum chamber, extremely long products can be easily treated with short processing times, immediately prior to gluing, bonding, or coating.

What materials and products can benefit from corona discharge?

Corona discharge treatments are highly effective on virtually any surface format including dimensional objects, sheets, and roll goods that are handled in a web format. Corona discharges are widely used to treat a variety of materials and products in the plastic film, extrusion, and converting industries, including:

  • Polypropylene
  • Polyvinylchloride (PVC)
  • Polyethylene terephthalate (PET)
  • Polyethylene
  • Vinyl
  • Paper
  • Paperboard stock
There are several basic corona treatment system configurations used for treating materials in web form. They are defined essentially by the location of the dielectric material in the station: conventional, bare roll, double dielectric, and convertible. The configuration that is best for a given application depends mainly on the material being processed.

Corona discharge surface treatment is an effective and efficient process for increasing the surface tension of a wide variety of materials, parts, and packaging in order to provide a surface that is more receptive to inks, coatings, or adhesives. Highly consistent and controllable, the process continues to be adapted for new applications using both standard and innovative materials.

To learn more about the use of plasma in manufacturing, please read our eBook titled "Manufacturer’s Surface Activation Guide for Improved Adhesion."




The effects of a corona discharge on surfaces

There can be a variety of effects on surfaces that are treated with corona plasma. It is known, for example, that fluoroplastics like fluorinated ethylene propylene (FEP) or polytetrafluoroethylene (PFTE) exhibit improved adhesion after the treatment. This allows painting or printing on such material, which would be much harder on non-treated surfaces. On natural fibers like wool, corona discharges can enhance wettability and prohibit the shrinking of the fabrics

In other experiments on polymers like low-density polyethylene (LDPE), this kind of plasma treatment leads to changes in the surface roughness and alters the hydrophobicity of the substrate. The big advantage is that corona plasmas can also be ignited at atmospheric pressure and in (partly) reactive gases such as air, oxygen, or nitrogen. Another important point is that there are relatively few ions or free electrons present. Thus, no actual electrical breakdown occurs and the surface treatment itself is quite mild.

Nevertheless, there is also the possibility that the surface chemistry itself is altered. It has been shown, for example, that polypropylene (PP) films incorporate oxygen into the surface layer, which then changes the wettability drastically.

Corona treatment of non- conductive and polymer surfaces can also lead to improving the adhesion between them. The range of polymers is quite wide. Improved adhesion was also reported for PEEK or PET.

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