Structural Separation
Multi-ply paperboard consists of separate fibrous webs that are joined under pressure at the wet end of the paper machine. When the adhesive bond between these individual plies fails under mechanical stress, interlayer delamination occurs, causing the paperboard to split into its constituent layers. This failure mode destroys the bending stiffness of the sheet and renders it useless for structural packaging applications.
The risk of this failure is present in all multi-ply materials, particularly those with high chemical pulp outer plies and high-yield mechanical pulp core plies.
Mechanical Load
Industrial converting operations subject the paperboard sheet to heavy shearing and peeling stresses during creasing and folding. If the ply bond strength is too low, the force of the creasing rule will initiate interlayer delamination instead of a clean, controlled fold. This failure occurs during high-speed folding when the board is forced through tight bends.
Careful dosing of starch at the wet-end spray system increases the chemical bonds between the wet plies to resist these converting forces.
Quality Verification
Standard testing uses the Scott Bond method to measure the energy required to split a sample of paperboard. This test applies a rapid dynamic impact to a specimen held between two double-sided adhesive tapes. Low test results warn the converter that the board is likely to fail during the gluing and filling stages.
