Interfibrillar Swelling Stress Redistribution Analysis for Coated Recycled Fiber Boards
Interfibrillar swelling in recycled board plies generates interface shear stresses that fracture coatings and degrade stacking strength under cyclic humidity.
Interface Shear Stress
Mechanical Friction
Internal forces acting parallel to the boundary between two bonded layers can disrupt the structural continuity of a composite material. In paper converting and lamination, interface shear stress represents the force per unit area that tries to slide a coating or a film along the surface of the base paper. This stress arises during winding, folding, or when the sheet undergoes bending.
Coating Adhesion
Coating adhesion must exceed the calculated shear forces to prevent delamination of the barrier film. If the adhesive does not wet the fiber surface fully, interface shear stress causes localized peeling at the edges of the carton. Packaging lines run more reliably when the bond between the plastic barrier and the paperboard is stronger than the internal cohesive strength of the paper fibers.
Deformation Resistance
Deformation resistance during the creasing and die-cutting steps depends on managing these localized stress concentrations. When a flat blank is folded into a carton box, the outer plies of the board stretch while the inner plies compress, generating interface shear stress at the boundary. Proper creasing dies distribute this force across a wider zone to prevent the board from splitting or bubbling.
Modern high speed converting machinery relies on precise tension control systems to minimize tension spikes that could otherwise initiate delamination. Stiff boards with low interlayer strength require carefully optimized fold profiles to survive the fast-moving folding section.