Moisture Interaction
The physical uptake and release of water molecules by the hydrophilic structure of wood pulp defines a fundamental mechanism governing paper performance. This cellulose fiber sorption occurs due to the abundance of hydroxyl groups on the polymer chains, which readily attract and bind water via hydrogen bonding. It dictates how a paper web behaves when exposed to varying ambient humidity.
Fiber Expansion
Moisture behavior of paper during printing or converting depends heavily on the rate at which water is absorbed or desorbed by the cell walls. This internal migration of water molecules causes the individual fibers to expand in width much more than in length. Because fiber alignment is typically oriented in the machine direction, the resulting expansion occurs primarily across the web.
This cross-direction change can lead to registration errors during multi-color offset printing where the sheet is exposed to dampening water.
Dimensional Control
Controlling the moisture history of the paper before it reaches the press is essential to prevent waviness and curling. If cellulose fiber sorption is left unmanaged during storage or transit, the sheet edges will adjust to the ambient air faster than the center of the pile. This differential moisture uptake results in wavy edges when the humidity is high, or tight edges when the air is dry.
Modern paper mills pack finished reels and reams in moisture-proof vapor barrier wraps to maintain a stable internal equilibrium moisture content until the package is opened.
