Ply Separation
Delamination of multi-ply paperboard is prevented by the internal adhesive forces that bind the distinct fibrous webs together during forming and drying. A high level of inter-ply cohesion ensures that the board behaves as a single structure when subjected to mechanical forces. This property is influenced by wet pressing and the application of chemical binders.
Hydrogen Bonding
When wet webs are brought together on a multi-fourdrinier machine, water removal draws the surface fibres of adjacent plies into close contact. Hydrogen bonds form across the interface as the sheet dries, which is the primary mechanism behind inter-ply cohesion. Spraying cooked starch or synthetic polymers between the plies before wet pressing provides additional chemical bridging.
This treatment is necessary when using recycled fibres, which have lower natural bonding capability than virgin pulps.
Mechanical Stress
Converting and printing subject the board to severe peel and shear forces. During creasing, the board must undergo controlled internal shearing to form a clean bead, and insufficient cohesion leads to messy splitting, cracking or tearing. Dynamic testing using the Scott Bond method measures the energy required to split the plies apart, with target values depending on the board grade.
Low cohesion forces a reduction in printing press speed to avoid delamination under the suction of high-tack offset inks, which can halt high-volume production.
