Secondary Fiber

Fiber Morphology

Mechanical recycling subjects cellulose fibers to repeated wetting, pressing and thermal drying cycles. Processing secondary fiber causes irreversible lumen collapse and loss of swelling capacity, a phenomenon called hornification. Recycled fibers exhibit lower inter-fiber bonding potential and shorter average fiber length than virgin kraft pulps.

Paperboard mills blend secondary pulp with virgin softwood kraft to maintain target burst strength and stiffness in folding cartons. Contaminants such as residual inks and stickies must be removed through flotation and screening prior to sheet forming.

Structural Capability

Boxboard plies made from recovered pulps provide bulk and stiffness at lower raw material costs. Multiple repulping cycles degrade individual fiber strength until short fines dominate the stock. Excessive fines content reduces drainage rate on Fourdrinier forming wires and press sections and lowers board tear strength.