Molecular Cohesion
Intermolecular attractive forces between hydroxyl groups on adjacent cellulose polymer chains govern internal cohesion within papermaking fiber mats. Formation of a hydrogen bond network provides the principal mechanism for dry paper strength and mechanical stiffness in paperboard substrates. The network develops during wet pressing and thermal drying as water evaporates, allowing cell wall microfibrils to come into intimate physical contact.
The structural bond domain excludes covalent crosslinking and mechanical fiber entanglement forces.
Tensile Strength
Mechanical strength in paperboard depends on the density and area of inter-fiber bonding points across the sheet matrix. Strengthening the hydrogen bond network through mechanical refining flattens pulp fibers and increases relative bonded area, boosting tensile index and burst strength. Chemical strength additives like cationic starch or polyacrylamide promote additional bonding sites between negatively charged fiber surfaces.
Fiber orientation during web forming creates anisotropic bonding properties, resulting in higher tensile strength along the machine direction than the cross direction. Refining intensity and wet pressing pressure directly control bonding density, enabling paper mills to tailor stiffness characteristics for specific packaging converting demands.
Moisture Degradation
Water molecules readily penetrate cellulose structures, disrupting inter-fiber bonds and softening paper networks under humid conditions. Atmospheric moisture weakens the hydrogen bond network, causing loss of box compression strength and structural creep during storage. Wet strength resins prevent bond dissolution when paperboard contacts liquid water.
