Dimensional Expansion
Volumetric expansion of fibrous substrates occurs when liquid absorption or relative humidity increases expand the cell wall dimensions of individual cellulose fibres. Within liquid packaging and aqueous coating applications, paperboard swell measures the fractional change in caliper resulting from fluid penetration into the web matrix. Water disrupts hydrogen bonding between adjacent cellulose microfibrils, pushing fibres apart along their radial and tangential axes.
Because longitudinal fibre expansion remains minimal, dimensional changes occur primarily as an increase in total sheet thickness. Uncoated cut edges absorb fluid rapidly, leading to edge wick phenomena in beverage cartons.
Converting Failure
Caliper increases alter mechanical clearance through converting equipment and cause registration errors during multi-pass printing operations. Uncontrolled paperboard swell induces web tension fluctuations that lead to sheet tracking failures in high-speed offset presses. In extrusion coating lines, edge expansion produces uneven coat weight distribution across the transverse profile.
Swelling around scored package folds compromises score line integrity and reduces top-load stacking strength in filled cartons. Moisture barriers applied to outer surfaces reduce fluid contact, but unprotected cut edges remain vulnerable during liquid filling operations.
Operational Limit
Substrate thickness changes cease once fibre cell walls reach saturation point. Beyond this absorption threshold, additional fluid fills inter-fibre capillary voids without causing further cell wall expansion. Sizing agents like alkyl ketene dimer limit absorption rates to delay dimensional distortion during converting.
