Fiber optic gel point sensor for paper coating drying control and optimization

In the production of pigment-coated paper or paperboard, the method and rate of drying of the coating will significantly influence the print quality of the finished product. Improper drying during initial st sages can cause binder migration that leads to its non-uniform concentration on the surface of the coating. Such effect causes print mottle, and is a primary reason for poor print quality. The location at which binder immobilization has occurred is called the Gel Point. An on-line fiber optic sensor system has been applied to continuously monitor the location of the Gel Point. Each sensor consists of a light source, an optical fiber for delivering the radiation, two optical fibers for diffuse and specular radiation monitoring and two detectors. The ratio R between diffuse and specular signals indicates coating conditions. A system description and experimental results are presented. The Gel Point measurement system has show that is provides a sensitive measurement of the gel point location, as it is affected by machine speed, coat weight, incoming substrate moisture levels and drying energy and can predict print mottle. This provides the paper maker with a completely independent method for monitoring the coating drying process.