- March 13, 2012, Kelly Robinson
corona treater or corona discharge treater (CDT) in Figure 1 typically is located just prior to coating because the treated surface is more wettable and treatment improves adhesion between the coated layer and the web. Static dissipaters have no effect on the treatment level. Corona treaters change the chemical composition by oxidizing the web surface. Static dissipaters provide oppositely charged air ions to neutralize static. Consequently, static dissipaters have no effect on the chemical composition of the web.The
Corona treaters produce several unwanted byproducts, including static charge on the treated surface, a layer of low molecular weight polymer on the treated surface, and ozone. Each of these byproducts should be controlled. Ozone is controlled by an exhaust system. Minimize the low molecular weight polymeric layer by limiting the corona treatment level to only what is needed for your product. Increasing the treatment level increases the low molecular weight polymeric layer.
Two static dissipaters are needed to control static because the corona treater in Figure 2 produces static in two ways. During operation, the high voltage power supply energizes the corona treater electrodes causing an electrical discharge between the electrodes and the insulated treater roller that oxidizes the web surface. This electrical discharge also deposits static charge on the web, making it the primary source of charging. The polarity of the static can be either positive or negative, and I have measured levels exceeding 40 μC/m2 on a web exiting a corona treater. The maximum possible charge on a free span of web is 27 μC/m2 because above this value, the electric field near the web causes air to ionize.
The corona treater in Figure 1 can deposit such a high level of static on the web that a backside discharge occurs at the exit nip where the web peels from the treater roller. Backside discharge is the secondary source of charging. This backside discharge deposits oppositely charged static on the untreated side of the web reducing the web charge to less than 27 μC/m2. And the backside discharge deposits static charge on the treater roller surface. Static on the treater roller is bad because it can cause the treater roller to fail from an arc through the insulating layer.
Static charge on the treated web surface is neutralized in Figure 2 by a static bar located about 2 in. downstream of the exit nip where the web peels from the treater roller. Locate the static bar near the exit nip to suppress the backside discharge. In some applications, the static bar may be replaced by an ionizing string, which provides satisfactory static control for robust coating methods like gravure coaters.
While the static bar located near the exit nip should suppress the backside discharge, it may not completely eliminate the discharge, so static still may accumulate on the roller surface. Static charge on the treater roller is neutralized by the static brush located near the bottom of the roller. While the static brush does not need to touch the treater roller, bristle tips should be no further away than the thickness of the insulating layer on the roller; ~ 1 mm.
Corona treaters are a major source of static charge. Two static dissipaters are needed to control static because corona treaters generate static in two ways. The primary source is the electrical discharge and the secondary source is backside discharge. Locate a static bar just downstream of the exit nip to neutralize static on the treated surface. And to extend the service life of the treater roller, locate a static brush to neutralize charge on the roller surface.
I invite you to ask questions about this column and to suggest future topics. My e-mail address is: Kelly.Robinson@ElectrostaticAnswers.com.