Fibre Expansion
Dimensional instability characterizes the response of cellulose networks when moisture penetrates the internal structure of paper. Z direction swelling measures this volumetric increase occurring through the thickness of a substrate as hydrogen bonds between fibres weaken and break upon liquid contact. Hydrophilic additives and sizing agents regulate the velocity of this process, determining the duration a sheet maintains structural integrity before losing geometric stability.
Moisture Response
Liquid penetration into a porous sheet induces mechanical strain because water molecules occupy the voids between individual fibres. This lateral and vertical growth causes cockling or blistering on the surface of coated stocks, especially when fountain solution or aqueous adhesives touch one side of the substrate. Calendering pressure during manufacturing controls the density of the fibre mat to mitigate the rate of absorption.
Presses encounter sheet registration issues when the expansion shifts the relative position of ink laydown. Rigid fibre bonding reduces the extent of this vertical displacement during saturation.
Production Limits
High porosity stocks demonstrate greater susceptibility to thickness increases compared to dense grades treated with heavy internal sizing. Coating weight dictates the barrier properties that delay water contact with the base sheet. Mills establish target thresholds for thickness change based on the specific end application where dimensional control remains a primary requirement for press performance.
Total thickness increase correlates directly to the amount of water absorbed by the cellulose matrix.
