Dielectric roll covering

Elimination of the dielectric roll covering, which required frequent replacement and entailed an inventory of spare rolls, as well as considerable production time iosses during replacement of a ground roll, which would typically burn out. 

With covered roll designs, one must consider that the treatment outcome is dependent in part on the amount of applied power required and the type of dielectric roll covering to be used for the particular application. Therefore, the performance characteristics of the array of dielectric roll coverings must be understood to prescribe the right dielectric solution, Figure 4.2. 

Figure 80 Types of treater for applying corona discharge (a) for non-conductive substrates, with an earthed base roll covered with a dielectric (b) for material of all types, with a bare, earthed base roll and the dielectric on the discharge electrode The main components are (1) Multi-fin electrode (2) Electrode tube in ceramic or quartz (3) Earthed base roll covered with dielectric (4) Bare, earthed base roll (5) Housing for the electrode assembly, enabling the extraction of ozone and cooling of the electrode (6) Path of material being treated...

CSPE have excellent combinations of properties that include total resistance to ozone excellent resistance to abrasion, weather resistance even in light colors, heat, flame, oxidizing chemical, solvents, crack growth, and dielectric properties. Also provide low moisture absorption, resistance to oil similar to neoprene, low temperature flexibility is fair at -40C (-40F), low gas permeability for an elastomer and good adhesion to substrates. Can be made into a wide range of colors. Use includes hoses, roll covers, tank liners, wire and cable covers, footware, and building products (flash, sealing, etc.). 

The most common roll covering is a silicone sleeve. These sleeves have good dielectric strength at 350-400 inVkW, which allows them to handle all basic corona treating applications (a lower rating indicates that more power can be directed at the roll). For roll diameters greater than 8 in, vulcanized silicone must be used. 

The dielectric layer for ceramic capacitors is formed by preparing a barium titanate paste that is rolled into a flexible film. The film is covered by conducting ink to form the electrode layer. Then the film is cut, stacked into layers, and the procedure is completed in a manner similar to that already described. 

There are three types of treating configurations (Fig. 14.8) and they all consist of the same parts including an electrode, an electrical insulator or dielectric, and a return path or ground. The differences among the three configurations are in the location of the electrode. In a conventional system (Fig. 14.8a), the web passes over a roll which is covered with insulating... 

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. 

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. 

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