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.
Washboarding
Surface Defect
Uneven skin shrinkage in corrugated board produces a wavy surface that follows the pattern of the internal fluting. This cosmetic and structural fault, known as washboarding, happens when the linerboard draws tight across the peaks of the flutes during the drying phase. This deformity reduces the print quality on the outer faces of corrugated shipping containers.
Defect Mechanism
Moisture from the aqueous starch adhesive penetrates the linerboard and causes the paper fibres to swell and expand as the board is glued. When the board dries, the liner board shrinks but is held firmly in place by the glue lines on the flute tips. This constraint forces the paper in the spans between the flutes to sag, creating a series of parallel troughs.
Using high starch application rates or applying excess water to thin liners increases the severity of the distortion.
Print Quality
Irregular surfaces cause uneven ink transfer and crushed flutes when the board is run through flexographic printing presses. Printers adjust the nip pressure of the press rollers to try to compensate, but this can reduce the compression strength of the finished box. Selecting a heavier liner board or using low moisture adhesives minimizes the wave effect.