Fibre Class
Chemical wood pulps derived from coniferous trees provide the primary structural foundation for high-strength paperboard grades. Long coniferous fibers from northern softwood species yield high tensile strength and tear resistance when processed through alkaline sulfate cooking and multi-stage bleaching sequence. Bleached softwood kraft pulp supplies the long-fiber reinforcement required in multi-ply folding boxboard and linerboard formulations.
It governs the internal mechanical network without contributing unbleached dark lignin residues that impair surface brightness.
Structural Mechanism
Delignification during the kraft process dissolves intercellular lignin while preserving cellulose chain length. Refining then collapses the hollow tubular softwood fibers into broad, flexible ribbons that form dense hydrogen bonds across the web. This fibrillar network distributes tensile stresses evenly across the sheet surface during converting operations.
Higher refining energy increases sheet density and burst resistance, though excessive action reduces tear strength through fiber shortening.
Tensile Limit
Web stability during high-speed printing and package forming depends directly on long-fiber proportion. Incorporating bleached softwood kraft pulp into middle plies elevates stiffness and bending resistance while preventing ply separation under deep creasing forces. The structural benefit reaches a ceiling when excessive softwood fiber content raises surface roughness, which disrupts gravure or flexographic ink transfer.
Formulators counterbalance this stiffness contribution by layering short-fiber hardwood pulps on outer surfaces to maintain printing smoothness. Lower tear strength limits the replacement of coniferous fiber with secondary or short-fiber alternatives in heavy-duty packaging applications.
