Absorption Mechanism
Liquid movement through the porous structure of paper or paperboard is governed by the spontaneous draw of fluids into the void spaces between fibres. This physical phenomenon, known as capillary sorption, occurs without the application of external pressure. The rate of uptake depends on the pore size distribution.
Fluid Transport
Pore structure and surface chemistry dictate the velocity of liquid penetration. When paper comes into contact with printing inks or aqueous coatings, the capillary sorption rate determines how quickly the liquid phase separates from the solids. A rapid draw prevents excessive spreading but can cause pinholes in the coated surface if the fluid is sucked away too quickly.
Liquid penetration slows down as the pore radius decreases or the liquid viscosity increases.
Coating Interaction
Industrial coating operations require precise control over the liquid absorption rate to ensure uniform barrier properties. Uneven capillary sorption across the web surface produces mottled print patterns and inconsistent dry film thickness. Sizing agents like alkyl ketene dimer are added during stock preparation to modify the contact angle and slow this drawing action.
The balance between fibre sizing and coating rheology ensures that the applied fluid stays on the surface long enough to form a continuous, defect-free barrier layer.
