Internal Failure Mode
Separation of the internal layers of a paper or board substrate occurs when the bond between fibres fails under lateral force. Cohesive shear splitting describes the rupture that happens within the thickness of the sheet rather than at the surface or the adhesive interface. This phenomenon typically indicates a weakness in the internal sizing or a deficiency in the hydrogen bonding between the pulp layers.
It remains a distinct failure from adhesive failure where the bond between two different materials breaks.
Mechanical Cause
High-speed printing presses and folder-gluers exert substantial forces that can trigger the breakdown of the sheet structure. When cohesive shear splitting occurs, the force of the liquid ink or the pull of an adhesive strip exceeds the internal strength of the board. Multi-ply boards are particularly susceptible if the plies were not properly integrated on the paper machine.
Refining the pulp longer increases the surface area for bonding and helps prevent this type of structural collapse during conversion.
Testing Protocol
Laboratory instruments measure the energy required to pull the sheet apart in the Z-direction or through a delamination test. If cohesive shear splitting appears at low energy levels, the batch is usually unsuitable for high-stress packaging applications.
