How to Select a Covering for your Corona Treater
The Purpose of Corona Treaters
Plastic films are non-porous and typically have low surface tension. Inks, coatings, and adhesives do not readily wet out or adhere. Corona treatment raises the surface energy of these materials in a way that is uniform and not discernible to the eye.
A corona treater station for treating polymer films consists of a high voltage and high-frequency power supply or generator, a linear electrode, and a grounded roller with a small air gap between the electrode and the roller. The corona discharge propagates from the electrode to the film surface.
How a Corona System Works
The high voltage and high frequency imposed on the electrode causes the air between the electrode and roller to breakdown into a plasma. Plasma is a state of matter in which some of the outer electrons are stripped off of the gas molecules making and electrically conductive and chemically active kind of soup. The plastic film is briefly passed through the plasma/corona which acts to chemically modify and micro-roughen the surface of the film in a way that increases its surface energy.
Coverings Used For Corona Treater Rollers
Dielectrics that are used on the grounded metal roller include CSM rubber (originally DuPont Hypalon), silicone rubber, epoxy and epoxy/fiber composites, glass, and ceramic. Some of the first dielectrics used on rollers were CSM and silicone rubber before epoxy coverings were introduced to make the grounded roller cover more durable.
With multiple options available for coverings, it is important to keep these pros & cons of each in mind when selecting your covering:
- Good Insulator
- Good Web Traction
- Affected by high humidity, reducing insulating properties
- Silicone Rubber:
- Excellent insulator but poor abrasion resistance
- High web traction allows the use of lightweight web-driven rollers
- Much more cut & wear-resistant than CSM/Silicone Rubber
While CSM, silicone rubber, and epoxy are considered the more affordable choice, it is important to keep in mind that these three options are all subject to repeated chemical attacks by the same corona that is treating the film. Over time, this corona can oxidize organic components in the rubber or epoxy turning them into carbon dioxide and water, leaving behind the non-reactive or inert components like mineral filters. These mineral filters will leave a dusty layer on the roller surface.
In extreme cases, this erosion effect can make grooves or pits
in the roller surface filled with powder.
- Glass & Ceramic:
- Typically half the thickness as the organic coverings
- Not affected by the direct chemical attack of the corona
- Excellent cut & abrasion resistance
- Can handle two times the wattage of rubber or epoxy coverings of the same surface area or the roller’s diameter can be reduced
- Longer product life
- Considerably more expensive than organic counterparts
While glass and ceramic are similar in their resistance to oxidation and the ability to handle more wattage, there are some important differences between the two. Compared to ceramic, glass has a lower impact resistance, lower thermal conductivity, and is more easily damaged by static electricity.
Which Roller Covering is the Best?
All roller coverings can produce excellent results in terms of the treatment level and uniformity if the corona system is properly tuned and maintained, but each type has its limitations. Silicone, CSM, and epoxy coverings are generally low in cost and produce good results for most applications. Epoxy is much more resistant to cuts and abrasions than the others, but they all can be attacked by oxidation of the corona. Corona power levels are somewhat limited because the materials are not thermally conductive.
High line speeds or difficult to treat materials may require power levels above what silicone, CSM, and Epoxy can safely handle. Ceramic and glass coverings can handle double the power levels if the rollers are air-cooled, and four times the power if water-cooled. Some technical people feel that ceramic also produces more uniform charge density in the corona because of its relatively high capacitance.
Deciding which of the covering materials are best depends on the specific requirements and demands of your application. If several coverings will work, then the costs and benefits of each have to be evaluated. However, certain applications that require specific properties be met, your choices are often limited. Other times the choices are very limited with certain applications that require specific properties.
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