Fibre Shortening
Quality reduction in secondary fibers describes the progressive physical and structural breakdown of cellulose fibers during repeated recycling loops. This recycled fiber degradation is primarily caused by the mechanical shearing action of pulping, refining, and cleaning processes. Each cycle of recovery and processing shortens the fibers and reduces their ability to bond with one another.
Strength Loss
Loss of bonding potential leads to a drop in the bursting and tensile strength of the paperboard. As recycled fiber degradation progresses, the sheet density increases while the bulk and stiffness decrease, making the board less suitable for heavy-duty packaging. Papermakers must blend in virgin fibers or add chemical dry-strength agents to compensate for this decline in intrinsic fiber strength.
Chemical Treatment
Chemical additives can restore some of the lost dry strength by creating artificial hydrogen bonds between the shortened fibers. Starch spraying and the addition of polyacrylamides help maintain the compression strength of recycled corrugated medium, which is necessary to prevent box collapse under high humidity. However, these additives increase the drainage resistance on the paper machine, which limits the production speed and raises energy consumption during the drying phase.
Mill managers monitor the freeness of the pulp to determine the degree of fiber breakdown and adjust their refining energy accordingly to balance strength and machine runnability.
