Secondary Fiber Tensile Network Degradation under Variable Transit Humidity
Recycled pulp webs suffer severe tensile network degradation under cyclic transit humidity due to moisture-driven hydrogen bond disruption and hornified fiber creep.
Sizing Degree
Liquid Resistance
Internal sizing degree measures the retardation of fluid penetration through paper and paperboard webs by evaluating the rate at which an applied aqueous solution migrates into the fibrous matrix. Hydrophobic chemicals added to the stock suspension or applied at the size press establish surface energy barriers that interrupt capillary action along cellulose microfibrils. Mill operators regulate this metric during papermaking to ensure that finished packaging stocks withstand prolonged contact with liquid contents without losing structural integrity or allowing moisture transfer across creased folds.
Surface Application
Alkyl ketene dimer and starch emulsions alter the contact angle between water droplets and the sheet surface to retard lateral wicking during offset printing or liquid packaging converting. Coaters apply these sizing agents to the web within the wet end or at the size press to adjust porosity and surface energy according to downstream converting demands. Converters monitor this property to prevent fountain solution absorption from softening paper substrates during high speed lithographic runs, because excessive moisture uptake causes register distortion and dot gain on multi color presses.
Absorption Thresholds
Standardized Cobb testing quantifies water absorption over specified time intervals to verify that finished paperboard meets the liquid barrier requirements demanded by carton manufacturers. Variations in refining intensity, filler content and drying temperature alter the spatial distribution of sizing agents and change the Cobb value across the manufacturing web. Mill laboratories reject reels that fail to maintain minimum penetration resistance thresholds because under-sized substrates absorb liquid adhesives prematurely during box erection and fail to form reliable seals on high speed packaging lines.