Fibre Surface Modification
Mechanical peeling and detachment of microfibrils from the primary and secondary outer cell walls of wood pulp fibres occurs during wet refining. The process generates fine, hair-like fibrils that remain anchored to the parent fibre surface, dramatically increasing specific surface area without severing the main cellulose spine. By exposing hydrophilic sub-microscopic fibrils and expanding contact surfaces, external fibrillation promotes hydrogen bonding between adjacent fibres during wet-web consolidation and drying on the paper machine.
This mechanical modification is governed by refiner bar geometry, plate gap clearance, and specific edge loading conditions during pulp preparation. The action ceases when excessive refining shear induces fibre cutting rather than surface peeling.
Sheet Consolidation
Increased specific surface area improves wet-web cohesiveness, enabling higher press section dewatering rates and improved wet-web runnability. As water evaporates in the dryer section, surface fibrils generate strong capillary forces that pull adjacent fibres into intimate contact, maximizing the density of cross-linked hydrogen bonds. This mechanism elevates key paperboard performance properties, including internal bond strength, tensile strength, burst resistance, and surface picking resistance.
However, extensive fibril generation increases drainage resistance on the forming wire, slowing production speeds and elevating energy consumption during vacuum dewatering.
Stock Preparation Control
Papermakers regulate the extent of external fibrillation by monitoring Canadian Standard Freeness or Schopper-Riegler drainage values alongside automated optical fibre analysers. Selecting low-intensity refining plates with narrow bars preserves fibre length while peeling outer wall structures, maximizing strength development in bleached kraft pulps. Over-refining must be avoided because excess fine generation lowers paperboard tear strength, reduces opacity, and decreases sheet bulk.
Balanced mechanical fibrillation ensures optimal structural stiffness and folding performance in high-specification packaging substrates.
