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Description
Mechanosorptive creep (MSC), the accelerated time-dependent deformation of paper under simultaneous mechanical load and cyclic humidity, is a leading cause of compressive failure in corrugated containerboard during transport and storage. Despite decades of research, MSC prediction remains challenging because the relative roles of fiber-level hygroexpansion and network-level bonding are still contested. This study systematically compares the cyclic-humidity creep behavior of four industrially relevant corrugating-medium pulps: ammonium-base neutral-sulfite semi-chemical pulp (A-NSSC), sodium-base NSSC (Na-NSSC), eucalyptus high-yield kraft pulp (HY-KP), and recycled old corrugated containerboard (RP). Laboratory handsheets were prepared under two controlled conditions: a fixed-freeness set (Schopper–Riegler 25°) and a fixed-density set (approx. 700 kg/m³), both achieved through PFI refining. This dual approach decouples fiber-property effects from network-bonding effects on MSC. Compressive creep tests were run under cyclic humidity (50% ↔ 90% RH) until failure, and hygroexpansive strain, sheet density, water retention value, and wet zero-span tensile strength were recorded for each pulp-beating combination. A-NSSC showed the highest MSC resistance across both sets, attributed to its low beating resistance and resulting ability to form dense, well-bonded fiber networks at moderate refining levels. Time to failure correlated strongly with sheet density (R² = 0.87 for virgin pulps), confirming that fiber-fiber bonding governs creep resistance. Hygroexpansive strain, by contrast, showed no positive correlation with MSC. Pulps with the greatest hygroexpansion (A-NSSC) in fact exhibited the longest creep lifetimes. This contradicts earlier reports of a direct positive link between hygroexpansion and MSC acceleration, and is explained by the well-known co-dependence of hygroexpansion on sheet density: denser sheets expand more yet resist creep more effectively through superior bonding. These results establish that network bonding quality is the dominant factor controlling MSC in corrugating-medium pulps, while hygroexpansion alone is not a reliable predictor. Practical improvements to creep resistance should therefore prioritize fiber-fiber bonding and sheet densification over minimizing hygroexpansion.