Thickness Expansion
Hygroexpansion measurements record the out-of-plane dimensional enlargement of paper and board networks upon moisture uptake. Identified in laboratory testing as zd swell, this dimensional change tracks caliper increase through the thickness axis perpendicular to the machine and cross directions. Swelling behaviour influences surface roughness and print register stability on water-based printing presses.
Stress Release
Wood fibres naturally expand far more across their diameter than along their longitudinal axis when absorbing ambient water. Because papermaking machines orient the majority of fibres flat within the horizontal plane, individual fibre radial expansion translates directly into bulk sheet thickness increases. Calendering compresses wet paper webs into dense uniform sheets, locking mechanical drying stresses into the internal network.
Wetting releases these frozen-in compaction stresses, causing unrestrained springback that permanently alters sheet caliper and smoothness. High-speed inkjet presses experience dot gain distortion and fiber rising when water-based ink carriers penetrate unsealed board surfaces. Multi-ply boards with divergent furnish recipes across top and bottom layers warp into curls as asymmetric swelling destabilizes panel flatness.
Hydrophobic Boundary
Internal sizing agents and hydrophobic top coatings stop transverse expansion by repelling liquid water droplets. Alkyl ketene dimer and alkenyl succinic anhydride chemicals prevent capillary absorption, restricting dimensional change until the surface barrier is physically broken.
