Material Morphology
Heavyweight paperboard manufactured from unbleached secondary fibres retains significant residual wood polymers within the recycled pulp matrix. High lignin recycled board incorporates recovered corrugated containers, old newsprint and mixed mechanical waste grades that carry substantial quantities of unmodified lignin. The presence of aromatic polymer networks within the fibre cell walls imparts natural stiffness, bulk and compressive strength to unbleached board plies.
Because lignin resists water swelling during repulping, fibres derived from mechanical wood pulps exhibit lower inter-fibre hydrogen bonding capability than bleached chemical wood pulps. Mills utilize multi-ply forming sections on cylinder or fourdrinier board machines to place these stiff furnishes in inner filler plies while reserving clean chemical fibres for white top liners.
Mechanical Behaviour
Compressibility and ring crush resistance govern the structural performance of corrugated packaging medium and solid board containers produced from this grade. The rigid nature of lignified secondary fibres delivers high flat crush resistance and bending stiffness at economical basis weights. Moisture sensitivity represents an operational trade-off, because unbleached lignin absorbs atmospheric humidity rapidly, weakening fibre-to-fibre cohesion under high relative humidity storage conditions.
Box compression strength declines noticeably when unlined cartons made from this substrate enter chilled warehouse chains. Converting lines run corrugating fluting made from high-yield recycled pulp through steam shower zones to plasticize lignin temporarily, permitting flute formation without web fracture on corrugator single facers.
Converting Limit
Surface roughness and dark pulp colour restrict high-definition process colour printing to outer bleached liners or require heavy base coatings. High lignin recycled board exhibits low surface energy, rapid ink absorbency and elevated dusting during high-speed rotary die-cutting operations. Knives and creasing rules must be adjusted with wider clearances to prevent liner delamination and flaking along scorelines.
Packaging manufacturers specify this substrate for transit packaging, industrial partitions, carton filler layers and secondary outer cases where raw compression strength outweighs print gloss, whiteness and surface smoothness requirements.
