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Humidity’s Effect on Treater Rolls and Film Treatment
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The effects of humidity on treater station and, in particular, treater roll performance have not been discussed or researched at length, however, converters and film manufacturers have long experienced a rise in downtime and an increase in treater roll failures as humidity increases. This phenomenon is not a coincidence and can be explained by understanding both the quality of treater roll coverings and the electrical circuit created when a treater station is on.
Humidity can cause numerous issues for film treatment including; unacceptable treatment levels, non-uniform treatment and line downtime. One of the most effective methods to counteract the effects of humidity is to use an inorganic, non-porous dielectric treater roll coating. Currently, the only inorganic, non-porous material used for treater roll coatings is glassed steel. For combating the effects of humidity the use of a Glassed Steel Treater Roll is the most effective coating option.
A non-porous surface on a treater roll is paramount in controlling the effects of humidity during the film treatment process. Any porosity in the surface of a treater roll is not only a potential absorption point for humidity, it can also be a resting area for cleaning fluids and liquids. Even if no humidity or other liquid finds its way into the porous surface, the porosity will remain a potential source for “spotted” backside treatment and/or inconsistent treatment effects. To avoid problems caused by humidity (inconsistent and non-uniform treatment levels) and to minimize the occurrence of backside treatment, a non-porous surface on the dielectric roll is critical. The diagram below demonstrates the difference between a non-porous treater roll surface and a porous treater roll surface.
To understand why porosity exists within most materials it is important to understand that there are 2 basic types of materials: organic and inorganic. Organic materials are materials that include carbon. Organic materials will degrade over time and will degrade faster in manufacturing environments as heat, ozone, cleaning fluids, etc. react with the organic surface and cause it to chemically break down and carbonize. Typical organic treater roll coverings include silicone, epoxy and hypalon.
Conversely, inorganic materials, which are materials that are mineral in origin, are not susceptible to surface chemical degradation. Inorganic materials can be particulate based or can be a fused or hardened liquid (think of molten lava which has cooled). Particulate based materials will still result in porosity, however, due to the nature
of the manufacturing process. An example of a particulate based inorganic material used for treater roll coverings is alumina oxide ceramic. Fused materials will result in a pore free surface. An example of a fused, inorganic material used for treater roll coverings is
glassed steel.
Glassed steel is a super cooled liquid coating. During manufacturing the coating is heated to high temperatures where it fuses to a uniform, pore-free, liquid glass insulating layer and all volatiles are released. As it cools and hardens it forms a surface free of porosity. Ceramic coverings are manufactured using a plasma spray process. The plasma spray process is a high temperature process that essentially sprays surface melted ceramic particles at high speeds onto the roller surface. Although the particles bond on their edges, voids remain between particles where the bonds are not formed. This results in porosity. These particles will also have a tendency to abrade or “flake-off” over time causing coating degradation.
These differences become important in understanding how humidity affects different types of treater roll coverings. As you might surmise, treater rolls with organic surfaces (silicone, hypalon and epoxy) are more susceptible to the effects of humidity. Although each coating has its own wear characteristics these surfaces will break down both chemically and electrically as they age and as they are used in manufacturing. These surfaces are also more likely to cause pinhole failures as humidity increases. The simple reason is that the organic surface has begun to deteriorate and has provided a location for moisture to collect or absorb. If you are fortunate, as a corona forms in the air gap between the film and the roll surface, any residual moisture left in the voids of the organic material will only cause a brief arc and not a pinhole.
Unfortunately, when an arc occurs it can further degrade the treater roll covering, “treating it”, eventually creating a carbonized region where increased levels of moisture may absorb. Eventually the coating will break down electrically in these areas and cause a pinhole or grounding track through the covering. Inorganic materials will last longer since they will not react with the climate or environment.
In order to achieve a corona discharge an efficient electrical circuit must be created. In a treater station this circuit includes a power supply, a metal electrode and a dielectic treater roll (which acts as a capacitor). This circuit is demonstrated in the diagram below:
Since the dielectric covered roll completes a capacitive electrical circuit, a pinhole in the dielectric covering will cause the capacitor, formed by the roll covering, to ground itself (short) and corona treatment will not be possible until the treater roll covering is repaired or resurfaced to restore the capacitive element in the treater circuit. In the simplest terms, the electrical circuit has been interrupted.
Obviously, any electrical breakdown of the capacitive element of the circuit will hinder the performance of this electrical circuit. Many facility and maintenance managers have had the unfortunate experience of having an inexperienced employee clean the treater roll yet have it fail (pinhole) on start-up. Sometimes this is caused by a conductive cleaning fluid that is left behind in the porous surface of a treater roll. When the treater station is turned on, and there are remnants of the conductive liquid, there can be an immediate grounding. Humidity can cause the same issue. In mild and high humidity environments humidity (water vapor) can find its way to the voids or porosity within a dielectric treater roll.
Humidity will also cause the efficiency of the treater station’s electrical circuit to decrease. As humidity surrounds the circuit, it is observed that it becomes more difficult to maintain the same dyne levels at the same power levels. Operators will increase the power input in order to maintain adequate treatment levels. Unfortunately, increasing
power levels beyond the dielectric strength of the treater roll covering will cause dielectric failure and again interrupt the electrical circuit. Choosing a roll covering which requires a lower power density (sq. inches of roll surface area/kw), will allow dyne levels to be achieved without dielectric failure. The treater roll covering that maintains the lowest power density and the highest dielectric strength is glassed steel:
In conclusion, humidity can have dramatic effects on both treater roll coatings and the treater station’s electrical circuit. Choosing an inorganic, non-porous treater roll covering requiring a lower power density, such as a glassed steel treater roll, will allow most converters and film manufacturers to reduce downtime and maintain adequate treatment even during the most humid time of the year.
Questions regarding this article can be directed to:
Eldridge M. Mount III
EMMOUNT Technologies
585 223-3996
emmount@earthlink.net
www.emmount-technologies.com
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