Structural Flattening
Microscopic changes in wood fibers occur when the hollow central cavities of the cells close permanently. During the papermaking process, cellulose cell collapse happens as water is removed and surface tension forces pull the cell walls together into a flattened ribbon shape. This transition is usually irreversible once the cell wall reaches a certain level of dryness.
It changes the fiber from a stiff tube into a flexible band that can form more contact points with neighboring fibers.
Bonding Density
Flattened fibers increase the total surface area available for hydrogen bonding within the paper sheet. Because cellulose cell collapse creates a denser and more compact network, the resulting paper gains tensile strength and internal bond strength. A sheet with high collapse levels is typically less opaque and more translucent than one with uncollapsed, tubular fibers.
Surface Uniformity
Smoothness improves when the fibers lay flat against each other rather than protruding from the surface. In packaging grades, controlled collapse is necessary to provide the level of finish required for high quality ink coverage. The process stops when the moisture content falls below the fiber saturation point, at which stage the cell walls are too rigid to further distort.
