Air entrainment bubble spots can also be created when the coating process is operating at high line speeds outside of the coating window. These entrainment bubbles are many small bubbles that are distributed across the entire substrate. They are caused by the boundary layer air on the surface of the substrate not being completely removed as the coating solution is applied. The remaining air film, forms many small bubbles that are distributed through out the coating.
In addition, bubbles can be trapped in the coating bead and can lead to bubble streaks.
Techniques for eliminating bubbles have been discussed in several publications 1-4. While these techniques do reduce bubbles, there are still many bubbles that can be present in the applied coating, which can be an ongoing source of defects.
Two relatively new online technologies are available to assist in reducing bubble defects on the coating line. They are in–line solution bubble detectors and surface inspection systems. They can reduce can reduce defect losses by early detection of bubbles in coating solution and coated film so that corrective action can be rapidly implemented.
The in-line bubble detectors analyze a flowing solution in pipe or tubing to determine of there are bubble in the solution line. They use ultrasonic technology to detect bubbles in pipe or tube that contains a flowing liquid. Figure 1 is an example of a typical sensor. The flow lines are usually plastic tubing and they are inserted into the detector. The detector can be inserted into the tubing between the solution delivery system and the coating applicator or between the debubbling process and the holding tank in the solution delivery system. When bubbles are detected the flow can be returned to the debubbling process so that bubbles are removed before being coated on to the substrate. When they are detected during coating, the sources can be located and removed, before they coating defects. A possible impediment to using these detectors is metallic tubing which would retard signal and prevent measurement. This could be avoided by having a section of plastic tubing for measurement systems. There are several.
When these bubble defects occur, they are difficult to detect on the substrate at typical line speeds. speeds and are usually detected by inspecting the dried film. This procedure is time consuming and can result in a significant quantity of defect film being manufactured until the offline inspection is complete. Also, quantitative data as to size, location and frequency of bubbles. This data is needed to remove the bubbles in the coating process and to locate bubble defect areas in the wound roll so that they can be removed and not sent to the customer.
The online surface inspections systems technology have improved so that bubbles of all sizes can be detected at coater line speeds and defect maps produced of their location. Figure 2 show a surface inspection system and images of bubbles that were detected during coater operation. Utilizing these systems will rapidly identify bubbles so that corrective action can be implemented. It will also permit online experiments, since the results can be immediately seen.
- Smith, R. D. The Ultimate Roll and Web Defect Troubleshooting Guide with Glossary, TAPPI Press 2013
- Cohen, E. D. Gutoff E. B, Coating and Drying Defects Troubleshooting Operating Problems, second edition, John Wiley and Sons, New York, 2006
- Cohen, E. Web Coating Defects: Reducing Bubble Spots, Converting Quarterly, 2011 Quarter 3
- Gutoff E. B, Eliminate Pesky Bubbles, Paper, Film & Foil Converter, November 30, 2004.