Dimensional Change
Structural alterations in paper sheets happen when individual plant fibers swell in response to changes in relative humidity. This cellulose fiber expansion occurs primarily in the radial direction of the fiber, which causes anisotropic movement across the paper sheet. The boundary of this physical change lies at the fiber saturation point, above which additional water does not cause further swelling.
Moisture Mechanism
Water molecules enter the disordered, amorphous regions of the cellulose polymer chains and push them apart by forming new hydrogen bonds. This action increases the volume of each individual fiber, translating into a macroscopic expansion of the entire paper web. In paper machines, fibers align more heavily in the machine direction, meaning that cellulose fiber expansion is significantly more pronounced in the cross-machine direction.
Paper mills control this behavior by optimizing the refining process, as heavily refined fibers swell more due to increased internal fibrillation and water holding capacity.
Pressroom Performance
Multi-color offset lithography demands tight register control to ensure that successive ink colors line up perfectly. When a paper substrate is exposed to dampening water on the press, cellulose fiber expansion can misalign the image by several millimeters. Operating plants run humidity-controlled pressrooms to limit this dimensional variance.
Dry sheets will absorb moisture from the air and expand, whereas overly damp sheets will release moisture and contract.
