Dynamic Vapor Sorption Mechanics in Recycled Packaging Substrates
Recycled packaging substrates exhibit non-Fickian moisture uptake and severe hysteresis, reducing compressive strength by over 20 percent under cyclic humidity.
Shear Stress
Force Metric
Parallel mechanical force applied per unit area tangential to the surface of a material measures the internal resistance opposing sliding deformation between adjacent planes. In paperboard converting, coating applications and corrugator bonding, shear stress governs fluid flow kinetics, adhesive joint stability and structural failure modes. The parameter is quantified in pascals or newtons per square metre, representing the ratio between the applied lateral force and the resisting cross-sectional area.
Cellulosic webs encounter continuous transverse and interlaminar stresses during web handling over curved rollers, through calender nips, during rotary die-cutting and under stacking loads on corrugated boxes.
Coating Rheology
Liquid formulation behaviour during blade, rod and curtain coating depends directly on the shear stress established within the fluid film under high machine speeds. Blade coating assemblies generate localized shear rates exceeding one million reciprocal seconds within the narrow gap between the metering blade and the backing roll. High shear stress causes dilatant pigment slurries to increase in viscosity, leading to blade scratching, coating streaks and web breaks.
Formulators introduce rheology modifiers such as carboxymethyl cellulose or synthetic associative thickeners to achieve pseudoplastic, shear-thinning behaviour, ensuring that coating viscosity drops predictably under high stress while rebuilding structure rapidly upon leaving the nip to prevent levelling defects.
Structural Delamination
Interlaminar failure in multi-ply folding boxboards and corrugated boards occurs when applied shear stress exceeds internal cohesive and adhesive bonding limits. During folding carton creasing, mechanical creasing rules press the board into female matrix channels, intentionally inducing controlled interlaminar shear delamination between plies. This deliberate internal shearing relieves bending tension in the outer liners, enabling crisp ninety-degree and one-hundred-and-eighty-degree carton folds without cracking the decorative clay-coated print surface.
Fluting adhesive joints in corrugated transit cases must withstand continuous cyclic shear stress during transport vibration, preventing glue-line fracture and flute uncoupling that catastrophically reduce dynamic top-load stacking strength. In-plane shear modulus measurements quantify the fundamental stiffness of paperboard plies under these multi-directional converting loads.