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.
Cell Wall Swelling
Structural Expansion
Liquid absorption during chemical pulping causes the compact internal structure of wood fibers to expand into surrounding liquor. This physical transformation, known as cell wall swelling, increases the internal volume of fiber walls while loosening the rigid network of hydrogen bonds between adjacent cellulose chains. Softened fiber walls become susceptible to mechanical refining without breaking.
Hydration Process
Alkaline liquors and mechanical beating break down internal crosslinks within the hemicellulose matrix, allowing water molecules to penetrate deep into the crystalline cellulose framework. As water enters these microvoids, osmotic pressure within the fiber wall rises, forcing the outer primary wall layer to split open. High cell wall swelling enhances pulp flexibility and conformability during wet web consolidation on the machine wire.
Refiners adjust chemical dosage and pulp slurry temperature to control hydration levels because over-swollen fibers drain slowly on the wet end, which reduces paper machine operating speeds and increases thermal drying loads.
Strength Boundary
Tensile strength and burst resistance improve when fibers flatten and form dense inter-fiber bonds during pressing. Excessive swelling degrades freeness and leads to crushing in the press section when water cannot escape the wet sheet rapidly enough. Controlled wall expansion balances sheet density against dewatering efficiency.