ABSTRACT
Once the fluid coating has been applied to the moving paper web, the paper passes through a section of the machine in which the excess coating is removed, often by a doctor blade. As the coating passes under the doctor blade, the paper adsorbs water from the coating. The paper then goes to a dryer section, where most of the water is removed to form a solid coating layer on the paper. If the coating’s water phase viscosity is too low or if the coating particles form large aggregates with a porous structure, too much water is lost prematurely from the coating under the blade. Solid aggregates form under the blade and scratch the coated paper. Excessive water loss also raises the coating solids until the coating becomes shear thickening (the viscosity increases with the shear rate). This causes undesirable streaking on the paper. Shear thickening occurs as a result of increased collisions between the colloidal particles in the coating. These collisions form aggregates that raise the viscosity. For good coating performance, it is important to control the viscosity the water phase and to suppress the tendency of the solid particles to aggregate upon collision. The viscosity of the water phase increases as the concentration of CMC increases.
