Part 1: Corona Treating for Coating Applications

This entry is part 1 of 5 in the series Corona Treating

Maintaining high quality coating on paper, film, metalized film and foil substrates at production line speeds requires a method of enhancing substrate surface energy. Corona treatment promotes bond sites and increases surface energy without sacrificing the positive properties of the substrate. In coating applications, treater system design has undergone significant changes to accommodate lighter webs, higher line speed operation and advanced substrates. The range of developments has included the requirement for handling conductive substrates, nips at the treater roll, using the treater roll as a “pull roll,” and modifying the treater station design to reduce wrinkling and “backside” treatment. Several techniques have been tried in search of good way to accommodate conductive substrates and to reduce wrinkling and backside treatment. One approach, dual dielectric/coated roll technology, has become by far the primary method used to overcome these problems. A history of the various technologies used is discussed, and the current state-of-the-art in equipment, control parameters, and applications are explained.

Equipment Basics: The Corona Treating Station

Figure 1 - Bare-Roll Station (ceramic electrode)

Figure 1 – Bare-Roll Station (ceramic electrode)

In the early eighties, a treating station was developed that revolutionized the structure of both the electrode and the station. The principal element of this revolutionary Bare-Roll design was the first successful movement of the dielectric covering from the treater roll to the treater electrode (Figure 1). This change included the first use of ceramic as the dielectric medium. Covering a roll with ceramic was not technically feasible at that time. This allowed the station to have an “open” design while still providing operators with safety from electrical shock. It also allowed the station to treat both conductive and non-conductive substrates. These benefits were well recognized at the time.

Less recognized was the simultaneous development of an electrode assembly that moved the location of the ozone exhaust from the perimeter of an enclosed station to the end of the electrode assembly. This new approach enabled the electrode assembly to rotate to allow the passage of web splices and knots. An enclosure was no longer required to capture and contain the ozone for removal. Instead the ozone was captured and removed immediately at the point of production in the air gap. This change was given little recognition at the time, yet provided significant benefits. It greatly reduced the opportunity for ozone to seep into the work area and, ultimately, permitted the construction of Covered-Roll stations in an open design.

Figure 2 - Bare-Roll Dual Dielectric Station (ceramic electrode, coated roll)

Figure 2 – Bare-Roll Dual Dielectric Station (ceramic electrode, coated roll)

The Bare-Roll Dual Dielectric Station is a recent development that expands the capabilities of the Bare-Roll Station. The Dual Dielectric Station (Figure 2), which uses a special ceramic coating on the roll as well as a ceramic electrode, matches or betters the treating efficiency and effectiveness of any other system.

With one exception, the Bare-Roll Dual Dielectric Station provides all of the benefits of the original Bare-Roll Station. The exception is the fact that the special coating may, but seldom does, require repair or replacement. However, unlike the traditional Covered-Roll Station, the Dual Dielectric Station does not become unusable when the dielectric coating is pitted, cracked or pin-holed. Since the electrode is also ceramic covered, the station can continue to treat. The dual ceramic provides an additional benefit in that heat stress buildup during operation is shared, and therefore neither electrode nor roll coating is subject to the same level of heat stress as that found in Covered-Roll stations. This, of course, increases the long term operational reliability of the coated roll.

The Bare-Roll Dual Dielectric Station provides an additional, highly significant benefit over Bare-Roll and Covered-Roll stations. The Dual Dielectric Station greatly reduces wrinkling of ultra light substrates and thereby also reduces undesirable “back-side” treatment, i.e., treatment on the side opposite of that for which the treatment is intended.

Part 2 of the Corona Treating Series discusses driving and nipping the treater roll.

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