Paperboard Middle Ply Furnish Compression Mechanics under Static Bench Conditions
Middle ply CTMP bulk provides structural moment of inertia, while short-span compression bench testing quantifies network collapse risks under load.
Fiber Wall Buckling
Structural Instability
Structural failure mode where the cell wall of an individual wood fiber collapses inward under axial compression. When paperboard is subjected to bending, fiber wall buckling occurs on the compression side of the sheet. Resulting microscopic deformation leads to a loss of stiffness in the overall structure.
Deformation Mechanism
Micromechanical properties of the fiber determine the threshold at which the cell wall loses its shape. Thin walled fibers from earlywood are more susceptible to fiber wall buckling than the thick walled fibers found in latewood. Permanent changes occur during this irreversible process as it marks the transition from elastic to plastic deformation within the paper matrix.
Increasing the refining level can change the fiber geometry and potentially alter the buckling resistance of the sheet. Chemical pulping methods that remove lignin often make the remaining cellulose structure more flexible and prone to this type of collapse. The humidity of the environment also plays a role as moisture acts as a plasticizer for the wood polymers.
Failure Outcome
Reduced box compression strength is the direct consequence of widespread cellular collapse. Once fiber wall buckling has initiated, the material can no longer support the same peak load as a pristine sample. The structural integrity of a carton depends on avoiding this failure point during stacking.