On-Line Measurement of Coating Color Quality, 1999 Coating Conference Proceedings

John-Erik Bergman & Ilkka Roitto

Valmet-Raisio Oy

Traditionally, the quality of coating color in the supply system has been controlled by physically sampling the coating color followed by laboratory analysis. The parameters analyzed are typically low shear viscosity, dry solids, pH and temperature.

Under present manufacturing conditions this is not satisfactory. The time delay is too long to react effectively to drastic quality fluctuations. Also, this traditional approach does not address the control issues relating to quality of the coating color in the supply system or coating kitchen.

When coater speeds were lower, analyzing coating color quality in the supply system was not critical. The papermaker had time to cope with the situation without the loss of large quantities of paper. As coater speeds increase and greater demands are placed on the coatings, continuous analysis of the coating color quality will be required.

Efforts to reduce coating color effluent and improve production efficiencies have also led the trend to integrate coating color preparation, supply system and coater into one process. This will increase the number of continuous coating color make down systems, which will increase the need for on-line analysis of the coating color. In a continuous make down system the time lag from coating mixer to coating head is only a fraction compared to a batch system. Again, continuous analysis of the coating color quality will be required to meet the environmental, quality and production goals.

Although some in-line instrumentation has been available, most of these instruments have been quite unreliable and inaccurate. However, instrument suppliers are intensively developing new systems to make real time measurements of coating properties practical. One example will be presented in detail.

At this first stage of instrument development, the increased availability of on-line analysis methods is limited to indication, alarm and information used for manual control. Other measurements such as temperature have already been utilized in control loops. Once a dependable method is developed for the measurement of dry solids and viscosity, it will surely be adapted for use with automatic control systems. The first systems for automatic solids control will be started in the near future. Viscosity control will require more time because of the very complex interactions between various parameters.