Evaluations of High-Speed Runnability Using Pilot Coater Data, Rheological Measurements, and Computer Modeling, 1993 Coating Conference Proceedings

John A. Roper, III, Jamel F. Attal

A variety of runnability problems such as blade weeping and scratching occur as paper coating speeds increase. These problems arise from a variety of underlying causes (basestock properties, coater head design, and formulation rheology) which are closely interrelated. This makes it difficult to predict runnability simply from laboratory rheological measurements. However, it is possible to correlate coating rheology with pilot coater performance for certain operating conditions which control the blade nip geometry and minimize blade wear.

Coating rheology data were collected using rotational viscometers for low- shear measurements and a capillary viscometer for high-shear measurements. The viscosity data were combined to generate a single flow curve for each formulation covering shear rates from 1 to 1,000,000 sec-1. This flow curve was fitted with a five-parameter model using an extension of the Cross equation to include dilatancy at high shear rates. This rheology model was incorporated into a computational fluid dynamics (CFD) simulation of a short-dwell coater which took into consideration differences in coater geometry. The blade pressures calculated from the computer model were compared with pilot coater data for the same formulation. It is hoped that by combining laboratory measurements with computer simulations, a better understanding of runnability can be achieved.