Fibre Length Loss across Recycling Loops and Score Cracking
Recycled fibre shortening and hornification limit bending strain, requiring wider matrix channels and virgin top layers to prevent score cracking.
ISO 15361
Tensile Parameter
Determination of the wet strength retention for paper substrates finds its formal specification in ISO 15361, which evaluates how effectively a web resists tearing when saturated with liquid. Water molecules break the hydrogen bonds holding cellulose matrices together, reducing breaking resistance until manufacturers apply wet strength resins like polyaminopolyamide epichlorohydrin during stock preparation. Mill laboratories submerge conditioned specimens in distilled water for specific immersion durations before mounting them on constant rate of elongation testers to measure residual force limits.
Lower values signal insufficient chemical cross-linking within the pulp slurry, leading to web breakage during high speed offset lithography printing or liquid packaging converting operations. Production standards mandate minimum retention thresholds because container boards destined for cold storage or agricultural display must withstand high humidity environments without suffering structural collapse or flange delamination.
Immersion Protocol
Standardized testing demands strict adherence to water temperature controls and exact soaking intervals because thermal variations alter fluid penetration rates into dense paperboard structures. Technicians clamp each soaked strip vertically between pneumatic jaws, applying tensile stress until rupture occurs while logging force displacement curves digitally. Specimen preparation requires climate controlled rooms maintained at exact relative humidity percentages to eliminate ambient moisture interference before liquid exposure begins.
High caliper folding boxboards require extended saturation periods to ensure complete water absorption through interior filler layers, whereas lightweight tissue grades achieve saturation almost instantaneously. Intermittent tension checks verify that clamp slippage does not distort breaking load data during high elongation phases.
Resin Chemistry
Chemical cross-linking density dictates the ultimate performance limit established by the protocol, determining whether a paper grade survives rigorous food contact applications. Polymeric additives form covalent bonds with cellulose hydroxyl groups, creating an insoluble network that restricts fiber swelling and subsequent bond rupture upon wetting. Excess resin addition causes sheet embrittlement, reducing tear resistance and complicating subsequent converting steps like die cutting and creasing.
Converters specify minimum wet tensile indices to guarantee that liquid packaging cartons maintain structural integrity during chilled transport and retail display stacking.