Modification State
Physicochemical modification happens when absorbed water molecules disrupt hydrogen bonding between cellulose microfibrils and hemicellulose within the fibre structure. Cell wall plasticization increases the flexibility of the individual fibres and reduces the stiffness of the resulting paper web. This transition from a rigid glassy state to a more compliant rubbery state occurs under high humidity conditions.
Softening Process
Softening happens as water acts as a lubricant at the molecular level, allowing polymers to slide past each other more easily. The hydrogen bonds that typically hold the amorphous regions of the cell wall together are replaced by bonds with water molecules. This process is reversible but lowers the elastic modulus of the board.
Converting operations must manage tension carefully because the material becomes more prone to permanent deformation when wet.
Converting Impact
Creasing and folding performance depend on the moisture levels of the stock at the point of impact. If the moisture content is too low, the fibres remain brittle and the surface cracks during the fold. Excessive moisture leads to a loss of score-line definition and reduced box compression strength.
Controlling the environmental conditions in the converting plant maintains the correct balance of ductility. High-speed packaging lines often include moisturizing units to ensure the board stays within the target window. Consistent fibre flexibility prevents structural failures during the formation of the carton flaps.
