A Time Dependent Change in Coating Viscosity: Improved Control of Viscosity Through Selection of Coating Components, 2003 Spring Technical Conference Proceedings

A North American mill observed an irreversible increase in coating viscosity with time in the supply system of their paper machine coater. Coating viscosity was within target ranges following preparation. However, with time and recirculation, increasing viscosity resulted in pumping pressure exceeding the limits of the equipment, resulting in costs associated with lost productivity and waste.

Data from large-scale experiments showed increasing coating viscosity with time and recirculation. These experiments; however, required production coating quantities and 24 to 72 hours to complete. A small-scale laboratory test was developed in which coating viscosity was measured continuously over time at an elevated temperature. Results from this test simulated observations made in production and large scale experiments.

A designed experiment was conducted using the lab test method to assess the relative contribution of the raw materials in the coating to the viscosity increases observed in process. Major contributing factors shown to influence coating viscosity from the designed experiment were insolubilizer, pigment “fines” fraction and insolubilizer/co-binder interactions. Formulation strategies based on these findings were implemented to control viscosity build.