Fibre Expansion
Pulp refining in papermaking starts with the penetration of water into the internal structure of cellulose fibres. This water intake causes interfibrillar swelling, which separates the microfibrils and increases the flexibility of the individual fibres. The process increases the surface area available for hydrogen bonding during paper formation and drying.
Without this expansion, the resulting paper sheets would lack the necessary density and tensile strength. This structural modification is a fundamental precursor to achieving high sheet density because it allows the wet fibres to conform to each other more easily during pressing.
Refine Mechanism
Mechanical shear in the refiner disrupts the outer primary wall and allows the secondary wall to absorb water. As the interfibrillar swelling progresses, the fibre diameter increases while the length remains relatively unchanged. This change modifies the drainage behavior of the stock on the forming wire.
Paper mills monitor the process to optimize energy use and achieve consistent pulp hydration.
Structural Impact
Increased swelling improves the mechanical strength of the paperboard but raises the shrinkage during drying. A paperboard with high swelling exhibits superior tensile index but may suffer from dimensional instability when exposed to changing humidity. Managing this property is critical for ensuring flat paperboard for offset printing.
