Role of Extensional Viscosity in Paper Coating

In this paper, the role of extensional viscosity in different paper coating processes was evaluated. Numerical models for the various coating application processes (blade coating, film coating, and curtain coating) were constructed to calculate the extension rate. Different rheological methods were employed to measure extensional viscosity versus extension rate for model coating colour systems. Pilot coater trials were carried out to study the performance of each model coating colour in curtain, blade and film coating. It was demonstrated that extensional viscosity of model coating colours depends on the rate of extension, and colours can be extension thickening or extension thinning. With the numerical calculation results of extension rate in coating application processes, the extensional viscosity test results of model coating colours were matched to their performance in pilot coater experiments. It was shown that increasing the extensional viscosity measured at the appropriate extension rate that exists in blade or rod metering increased blade or rod load. Misting in film coating was reduced by low extensional viscosity measured at the appropriate extension rate. Cratering in curtain coating was reduced by increasing extensional viscosity measured at the appropriate extension rate. It was further concluded that the extensional viscosity can be used to predict the coating performance only if it is measured at the prevailing extensional rate of the specific coating process speed