Structural Collapse
Intramolecular bond formation in recycled wood fibers results in a permanent reduction of their water absorption capability. The process of cellulose hornification occurs during sheet drying when the removal of water forces adjacent cellulose microfibrils to lock together via hydrogen bonds. This collapse of the internal pore structure cannot be undone by simple rewetting.
Bonding Loss
Flexibility reductions in recycled fibers alter the papermaking characteristics of the pulp. Cellulose hornification decreases the surface area available for fiber-to-fiber contact in the subsequent sheet. Consequently, recycled sheets made from these fibers show lower tensile strength and higher bulk than sheets made from virgin pulp.
This change forces mills to alter refining strategies or use chemical additives.
Fiber Recovery
Mechanical treatment in refiners can partially restore the hydration potential of hornified pulp by peeling away the deactivated outer layers of the fibers. This process increases the flexibility of the fibers and raises the bonding strength of the recycled paperboard. Intensive refining, however, shortens the fibers and reduces the tearing resistance of the finished board.
Wet-end chemicals, including cationic starch and carboxymethyl cellulose, are added to the stock to compensate for the lost natural bonding sites without damaging the fiber length.
