Moisture Response
Dimensional change resulting from moisture sorption causes differential swelling along the preferential fibre alignment axes of paper webs. Hydrophilic cellulose fibres expand significantly in diameter while changing minimally in length, driving hygromorphic deformation across the sheet structure. Moisture absorption during printing or converting operations causes cockling and register errors in multi-pass offset presses.
Dimensional Instability
Anisotropic expansion differential between machine direction and cross direction creates internal stress concentrations within laminated paperboard. When relative humidity fluctuates, hygromorphic deformation generates asymmetrical dimensional shifts in multi-ply folding boxboards. Coated boards experience differential expansion between the stable mineral coating layer and the responsive fibrous base, resulting in diagonal twist or curl.
Moisture gradients through the sheet thickness induce curling towards the drier surface as hydrogen bonds contract unevenly. Restraining web tension during drying reduces cross-direction shrinkage, though residual stresses remain locked inside the sheet structure until exposure to high humidity releases them. Environmental conditioning of paper reels before converting aligns internal moisture levels with pressroom atmospheric conditions, mitigating sheet distortion.
Structural Limitation
Equilibrium moisture contents dictate the boundary beyond which permanent structural damage occurs. Severe hygromorphic deformation ruptures internal fibre bonds, permanently weakening the bending stiffness of corrugated containerboards. Storage in unconditioned warehouses subjects stacked paper pallets to humidity cycling, degrading stack stability.
