Mechanical Refining Impact on Multi Ply Boxboard Z Direction Tensile Development
Controlled low energy refining of middle ply mechanical pulp yields superior Z direction tensile development without caliper loss in multi ply boxboard.
Fiber Fibrillation
Mechanical Alteration
Refining equipment subjects papermaking pulps to mechanical shear forces that peel back the outer primary cell wall and expose sub-microscopic structural elements. This structural peeling, called fiber fibrillation, creates thin tendrils attached to the main fiber body, dramatically increasing the surface area available for hydrogen bonding during sheet formation. Paper sheets produced from fibrillated pulp display higher burst strength and tensile energy absorption.
Refining Action
Disc refiners push high-consistency pulp through narrow gaps between rotating patterned metal plates to induce intense compression and shearing actions. Internal shearing separates internal cell wall layers while external mechanical impacts detach microfibrils from the fiber surface. Proper fiber fibrillation balances internal flexibility against external surface roughing to optimize sheet density.
Operators measure freeness and drainage resistance to monitor the degree of mechanical treatment because excessive shearing converts valuable long fibers into short fines. Microscopic inspection confirms whether mechanical action created fine surface hairiness or caused detrimental transverse fiber cutting.
Dewatering Limit
Extended surface area retains water molecules through surface tension and capillary action within the exposed microfibril network. Sheet consolidation slows on the paper machine wire when fibrillation becomes excessive, increasing steam consumption in the dryer section. Balanced mechanical treatment preserves drainage capacity while maximizing dry strength properties.