Paperboard Repulpability Assessment under Standard Laboratory Disintegration Conditions
Standard laboratory disintegration assesses paperboard repulpability by measuring defibering rates, fiber yield, and screen reject fractions under fixed shear.
Freeness Drop
Drainage Variation
Slurry drainage resistance increases as wood fibers undergo mechanical refining to develop their bonding potential. The freeness drop calculates the reduction in drainage rate as measured by standard testing instruments like the Canadian Standard Freeness tester or the Schopper-Riegler apparatus. This reduction correlates with the intensity of refining and the physical changes in the fiber walls.
Monitoring this variation ensures that the pulp achieves the required bonding capacity without compromising wet-end drainage.
Refining Progression
Repeated passes through mechanical refiners flatten and fibrillate the wood fibers, which dramatically increases the surface area exposed to water. The freeness drop becomes steeper with each pass as the fibers become more flexible and conformable. This flexibility allows them to pack closer together on the forming wire, creating a denser sheet with higher tensile strength.
However, the tighter sheet structure creates a tortuous path for water to escape, requiring higher vacuum pressure to achieve the same dryness. In cardboard manufacturing, a high rate of water release is essential to maintain high production speeds.
Operational Window
Papermakers must operate within a narrow range where strength gains outweigh drainage losses. When the freeness drop goes too far, the stock runs wet and the sheet cannot be dried efficiently.