Bare roll

Figure 7.8 Configurations of corona treatment equipment. (a) Conventional configuration, (b) Bare-roll configuration.

It was also recognized that none of the metal electrode/covered-roll configurations would work when treating conductive substrates such as foils or metallized film, and that a very complex form of electrode would be required. The early bare-roll type systems consisted of a metal ground roll and a series of dielectric covered electrode rolls that were motor-driven. This was a very expensive, cumbersome method of treating conductive substrates, but it was the only method available. 

In 1980, a new configuration of electrode capable of operating on a bare roll was engineered. It surmounted the basic problems that were experienced up to that point. This electrode achieved the following operational goals ... 

Bare roll surface treating is best described as an air plasma, or corona, discharge treatment of the surface of continuous, flexible two-dimensional substrates, which are conveyed over a roll with no dielectric covering and beneath one or more electrodes with an appropriate dielectric (insulative) coating, see Figure 4.1. 

Bare roll treater rolls are typically specified to have smaller diameters than those of covered roll systems, because of the area of dielectric required to dissipate the heat generated in the corona. With bare roll systems, the diameter is dictated by the length of the roll (driven by the maximum material width to treat) and the number of electrodes required (specified by the Watt density requirement). This dielectric-less ground roll design is always paired with dielectric-surfaced (typically ceramic) electrodes to promote uniform corona discharging to both conductive and non-conductive material surfaces. 

The advantages of using universal ceramic-coated roll systems versus bare roll system are ... 

The corona discharge, particularly bare roll and universal roll designs, and flame treatment at atmospheric pressure have both been effective in improving adhesion of various substrates on extrusion coating lines (the image in Figure 5.3 shows a multilayer system). 

The trial runs were performed on a bare roll corona treatment system as well as on an atmospheric plasma treatment system, resident at the same pilot line. For all runs, two PVdC-coated polyester films were post-treated, one in the APT system with a helium/oxygen plasma, the other by means of a corona treater. Another sample constituted the control reference and was not treated. The samples were then printed at 90 mpm on a PCMC press with aqueous ink (Flint Ink) using a laser-engraved anilox roll to 300 lines per inch (Ipi) and a cell volume of 4.8 billion cubic microns (bcm)/sq.in. The printed image provided solid (100%) ink coverage. The calculated ink transfer (thickness) to the film, based upon this cell volume, was approximately 1.9 microns. The printed web drying temperature occurred at 121 °C, with a web temperature of 77 °C. 

Classic corona Bare roll Corona discharge Low frequency... 

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