Structural Expansion Characteristic
Cellulose filaments undergo dimensional enlargement when water molecules penetrate the internal amorphous regions of the polymer matrix. Fiber wall swelling occurs when hydroxyl groups attract and bond with liquid phase moisture or vapor, causing the structural lattice to push outward. This increase in thickness reduces the void space within a paper sheet, which alters local density and influences the bending stiffness of the finished substrate.
Surface coatings applied to the sheet often restrict this movement by filling exterior pores.
Dimensional Stability Influence
Variations in humidity control the equilibrium moisture content and the degree of internal expansion within the pulp structure. When fiber wall swelling occurs during the drying phase of papermaking, the drying tension interacts with the physical growth of the individual filament to generate internal stresses. These stresses dictate the tendency of a sheet to curl or distort when exposed to uneven moisture levels.
Mills monitor these internal pressures to ensure that the sheet maintains flatness throughout the converting process. Precise regulation of the refining process limits the initial susceptibility of the raw material to water uptake.
Chemical Treatment Consequence
Certain sizing agents and internal additives prevent excessive liquid penetration by creating a hydrophobic barrier at the boundary of the individual filament. Without these additives, fiber wall swelling allows for increased permeability that compromises the ink holdout and print registration of coated stocks. High levels of moisture absorption lead to surface picking where the bond between the fiber and the coating fails under the pull of high tack inks.
Proper management of this property ensures consistent performance on high speed offset presses.
