Structural Alteration
Industrial recycling of paper and board involves chemical and physical changes that reduce the swelling capability of cellulose fibers during subsequent papermaking cycles. This phenomenon, known as recycled fiber hornification, occurs when the fibers are dried, causing the internal pore structure to collapse and form irreversible hydrogen bonds. It decreases the flexibility of the fibers and alters their ability to conform during sheet formation.
This alteration affects the strength of the recycled paperboard produced from these recovered materials. It represents a major challenge for mills striving to maximize the recycled content of high-performance packaging.
Rewetting Resistance
Wetting the hornified fibers during the pulping stage does not restore their original water retention value or mechanical properties. The collapsed cell walls resist water penetration, which prevents the fibers from swelling to their original volume. This resistance reduces the surface area available for fiber-to-fiber bonding.
Bonding Loss
Reduced bonding between the fibers decreases the tensile and burst strength of the paperboard. To counteract this loss of strength, papermakers must increase refining energy or add dry-strength agents to the pulp. This action helps to restore some of the lost surface area and bonding potential.
The process allows recycled fibers to be reused several times before they become too short for papermaking.
