Dimensional Sensitivity
Fractional change in the physical dimensions of a fibrous substrate per unit change in relative humidity governs the registration accuracy of multi-pass printing processes. This physical property, known as coefficient of moisture expansion, describes how much a paper or paperboard sheet expands or contracts as it adsorbs or desorbs water vapor from the surrounding atmosphere. Controlling this behavior is critical in offset lithography where sequential colors must align perfectly to prevent image blurriness.
Anisotropic Response
Fiber alignment during the papermaking process causes this dimensional change to vary according to direction. The expansion is always greater in the cross-machine direction than in the machine direction because cellulose fibers swell primarily in their diameter rather than their length. This directional difference requires printers to align the long axis of the sheet with the cylinder of the press to minimize register errors.
Structural Impact
Changes in moisture content can generate internal stresses that lead to fluting, curling, or warping in corrugated board and laminated packaging. When paper is bonded to an expansion-resistant film or metal foil, any fluctuation in humidity will force the paper to swell while the barrier remains stable. This mismatch produces a curling effect that can jam automatic packaging machines or compromise the appearance of the retail carton.
Paperboards intended for moisture-variable environments are often treated with sizing agents or polymer coatings to slow down the rate of moisture exchange and stabilize the substrate.
