One of the weakest components in current corona treating equipment is the dielectric. This dielectric can either be in the form of a roller covering or a coated electrode. Repairing coated electrodes, in most cases involves simple replacement, whereas repairing roller coverings have been done for years. Regardless of whether the coating is a Hypalon, EPDM, Silicone (sleeve or Bonded), Epoxy or Ceramic, patching a pinhole in the coating is possible.
It is recommended that a non-conductive material, such as an epoxy or unfilled silicone be used for the patch. The key is to try and select a material that will fill the hole with minimal porosity, adhere to the dielectric and have minimal shrinkage. For the purpose of this article, we will reference Jemmco JemmPatch two-part epoxy patch kit as the patching material.
Step one is to identify the hole, which with most dielectrics is easy. However, it can be difficult with ceramic coverings, as the hole may not have a burned area around it.
As implied, dielectrics other than ceramic will usually have a visible hole surrounded by burnt black areas. To identify the less obvious holes in ceramic, it may be necessary to use a handheld pinhole tester to identify the area.
Once the hole or holes are identified, the next step is to clean the hole, making sure all of the burnt areas are removed, as these areas are now most likely conductive, due to the presence of carbon. When patching ceramic, it will be necessary to use a drill with a diamond drill bit, in order to clean out the hole and to remove all of the surrounding contaminants.
In the perfect theoretical world, the hole should be shaped like a cone, so that the patching material can flow down one side of the cone, forcing the air up and out of the hole.
Once the hole is cleaned out, carefully clean and dry all of the surfaces to be bonded. Using the JemmPatch, remove the clamp and thoroughly mix the adhesive system components in the mixing-dispenser package until the color is uniform.
Apply the mixture to the prepared surface, trying to reduce the amount of entrapped air in the mixture and at the points between the mixture and the dielectric coating.
It is necessary to allow the patch to cure. In the case of JemmPatch, it will require 24 hours to fully cure at room temperature. In most situations, applying a good dielectric tape over the patched surface will enable earlier operation. JemmPatch will fully cure in 4 hours at temperatures of 150°F, which is a typical treater roll operating temperature.
If the tape is causing a backside treat, it can be removed after the adhesive is fully cured.