Molecular Transition
Molecular transitions within the cellulose and hemicellulose structure occur when absorbed water molecules disrupt the hydrogen bonding between polymer chains. Hygromechanical plasticization reduces the stiffness of paperboard by allowing the fibres to slide past one another more easily under stress. This state is reached when the relative humidity of the environment climbs above sixty percent for an extended duration.
Physical properties like the elastic modulus drop as the moisture content increases, making the material more pliable and less resistant to compressive loads.
Load Reduction
Structural integrity of a shipping container depends on the board remaining in a glassy, rigid state rather than a plasticized one. Once hygromechanical plasticization takes hold, the load-bearing capacity of a corrugated wall can fall by half. Creep deformation accelerates under these conditions, leading to the eventual collapse of stacked pallets in humid warehouses.
The transition is reversible upon drying but often leaves permanent dimensional distortions in the fibre matrix.
Barrier Performance
Coating barriers attempt to slow the rate of moisture ingress to prevent this softening. Wax or polymer films provide a temporary shield but cannot stop the eventual equilibrium with the surrounding air. Selecting fibres with lower hemicellulose content can provide a slight advantage in high-humidity environments.
