Hidden problems in surface treatments - II - Ground Rolls

To clean the ground roll is useful to prevent substrate damages during the treatment.

HIDDEN PROBLEMS IN SURFACE TREATMENTS - II - GROUND ROLLS

Rory Wolf, Enercon Industries Corporation, Menomonee Falls, Wisconsin, USA
Amelia Sparavigna, Dipartimento di Fisica, Politecnico di Torino, Torino, Italy


Lines under title: The quality of corona treatment strongly depends on the ground roll surface. Let us discuss the possible choices.


Is corona treating technology simple? This apparently seems a simple question. In fact, until a few years ago, the discussion of corona technology for surface treatment could have been limited to just two types of corona treating systems: covered roll systems and bare roll systems. But nowadays, new ceramic-coated ground rolls are enriching the technology and revolutionising the corona systems.
The quality of corona treatment strongly depends on the ground roll surface. This surface has a certain level of resistivity, depending on the existence of a covering over the discharging electrode. The material, which is being conveyed over the ground roll, is also important. A different voltage will exist as different materials pass through the corona discharge and over the ground roll. These variations in load can drive voltages high enough to damage the material being treated (Fig.1).

The ceramic-coated ground roll systems are able to provide more power than their predecessors could and also eliminate common problems associated with covered rolls. While the cost of a new silicone roll covering is not great, it is necessary to realise that there are more costs associated with replacing them, than just the cost of the new sleeve.
A silicone covered roll becomes damaged from either over-exposure to corona or because it was damaged by sharp devices. The damage can be severe enough to cause a high voltage trip (it may be possible to hear a "snap" or see an arc). In this case, it is necessary to shut down the production line, because this event occurs during a treatment run, to replace the roll covering. Of course, the damage may not be enough to cause a high voltage trip. The producer does not realise the existence of the damage until the production run is completed. The damage to substrates consists of pinholing or backside treatment. These two problems are often difficult to detect and certainly compromise the final product quality. Costs of waste time and production can quickly add up to more than just the cost of the roll covering.
Backside treatment occurs when air is trapped between the backside of the web and the ground roll. The air beneath the web becomes ionised and corona treatment occurs. When corona discharge occurs, the total energy in the air gap will be distributed between the top and bottom of the web. With a portion of the total energy applied to the undesired bottom surface, the power amount applied to the desired top side will be reduced. Generally, the treatment on the top surface of the material will decrease. The primary cause of backside treatment can be attributed to a lack of station cleanliness, specifically a dirty ground roll. Imaging the web contacting the surface of the ground roll, it is clear that any high point or build-up on the roll surface potentially lifts the material off the ground roll face. The immediate area surrounding the point is no longer contacting the roll face, resulting in an air gap. The air gap beneath the material allows corona to be generated. Reducing the accumulation of dirt or residue on the surface can substantially reduce the conditions for backside treatment to occur. Other causes of backside treatment are wrinkling and slippage of films. Slippage of materials on the ground roll results in a laminar airflow layer underneath the web. A possible corrective action is obtained applying a nip roll to ensure the material positively contacts the roll face. In applications where low tension or thin materials are involved, nipping may cause other material problems. In these cases, it may be necessary to choose other solutions.

The rolls.
With the ceramic-coated ground roll systems, the corona treatments enter in the era of atmospheric pressure plasma treatments. It is possible to choose among several treating systems, the most recent are the Universal Roll systems. Let us conduct a short review of the definitions of all types of surface treatment roll coverings (silicone sleeves, epoxy-coated rolls, glassed steel rolls, ceramic covered rolls, conductive ceramic coated rolls, universal ceramic coated rolls and atmospheric plasma rolls). Before starting the description, let us remember an important parameter, the dielectric strength of a barrier, which is the ability to withstand excessive voltages. The electrode's wall thickness covering material is determined by its strength. The lower the dielectric strength, the thicker the wall must be. A thicker wall requires more power to produce an efficient corona. The unit measure of dielectric strength is V/mil (1 cm = 393.70 mil).

Silicone Sleeves: the most common roll covering is a silicone sleeve. These sleeves have good dielectric strength (450 V/mil), which allows them to handle all basic corona treating applications. Silicone sleeves are very economical and replaceable. The downside is they can be damaged from over-exposure to corona discharge or accidentally by the slip of an operator’s device. While the cost of a new silicone roll covering is not great, costs from unexpected downtime and comprised product quickly add up. Life expectancy of a silicone sleeve is six months.

Epoxy coated rolls: epoxies coated rolls offered a more durable covering than silicone and are also a good choice for basic corona treating applications. The dielectric strength is comparable (450 V/mil). Epoxy coatings have harder surface than silicones or rubbers and many converters select them as added insurance against unexpected roll failure from exposure to corona or accidental damages. Life expectancy of an epoxy covered roll is one year.

Glassed-steel rolls: these rolls provide the highest dielectric strength (800 V/mil). According to these roll covering suppliers, these coverings provide an energy savings and has a high efficiency which results in improved treatment levels. In most cases the roll diameter for a glassed-steel roll will be smaller than comparable roll coverings.

Ceramic Covered Rolls: these rolls are covered with a thick ceramic covering (1.5 mm to 2.5 mm) that is applied via a plasma-spray technique. This roll covering replaces the rubber or epoxy coverings used with metal electrodes. The purpose of the roll covering is to provide a dielectric buffer between the metal electrode and the roll covering allowing the air to ionize and create the plasma corona in the air gap. Ceramic roll covering is widely used because of its resistance to physical damage. This type of electrode / roll combination cannot be used to treat conductive films.
These rolls are ideal for high speed treating applications and offer exceptional durability (two year limited manufacturer’s warranty). Usually the initial cost of a ceramic covered roll is greater than a silicone covered roll, but a cost comparison over the life of the station can work out favourably for a ceramic roll covering.

Conductive Ceramic Coated Rolls: conductive ceramic is a thick coating (0.05 mm to 0.13 mm), applied via a plasma-spray technique (Fig.3). The coating protects the ground roll from oxidation and corrosion. This covering replaces those chrome or nickel plating which are porous and will allow oxidation between the roll core and the plating. From an application standpoint, a conductive ceramic coated ground roll in a bare roll system is recommended when treating metal foils or the non-conductive side of conductive films. The conductive ceramic coating ensures a conductive path to ground from the conductive web surface. The primary benefit of the conductive ceramic is to prevent oxidation to the metal roll surface.

Universal Rolls: the universal ceramic-coated roll has a proprietary ceramic coating, applied to the roll core via a plasma-spray process. The universal ceramic coating is non-conductive and also serves as a dielectric covering on the ground roll. It is possible to use a universal roll system (Fig.4) to treat metallised films or foils. Because the high voltage electrode and the ground roll are covered with a dielectric, the corona is established in the air gap between the electrode and metallized film or foil. It is important that the metallized surface of the film be in contact with a clean and grounded idler roll. This idler roll can be covered with conductive ceramic to provide a ground path from the conductive film surface.
The universal ceramic-coated roll system provides superior treatment levels, with elimination of film wrinkling and backside treatment. It eliminates also the pin-holing on the metallized surface. Moreover, the higher efficiency ceramic covered roll requires less power to meet the same treat requirements.

Atmospheric Plasma Ceramic Coated Rolls: atmospheric plasma ceramic is applied using a plasma-spray process similar to Universal rolls. Although many aspects of this technology are proprietary, its primary advantages against all other ceramic-based roll coverings are described in the Table.


Atmospheric Plasma has
Highly uniform and homogenous glow discharges
Very high treatment levels
Long-lasting treatment
Ability to treat highly crystalline and engineered films,
wovens and non-wovens
Elimination of film wrinkling and puckering, backside treatment,
pin-holing on films and metallized surfaces


Upgrade of roll coverings
Changing the type of roll covering could benefit several treatment operations. However, it is necessary that the surface treater manufacturer determine how a change in roll covering may affect the rest of the surface treating station. For example, a change in the roll covering surely results in an alteration in sizing of the roll diameter. It is then necessary to determine the effect of new geometric relationships between electrode and roll. The new mechanical device must adequate to electric framework of the treater, to create a plasma discharge between electrode and ground roll. On new installations, the choice is much simpler, because it can be managed according to the main production requests.