Standard Laboratory Methods for Paperboard Flexural Rigidity
Paperboard flexural rigidity depends on the cube of caliper, requiring exact ISO 2493 or TAPPI T 489 instrument alignment and strict 23°C/50% RH conditioning.
Bleached Kraft
Chemical Brightening
Wood pulp undergoes intensive delignification and subsequent oxidizing chemical treatment to produce bleached kraft, a strong virgin fibre substrate utilized extensively in packaging and graphic paper manufacturing. Elemental chlorine free sequences or total chlorine free sequences remove residual lignin from the cellulose matrix without destroying the carbohydrate structure. Suppliers deliver this high brightness pulp in bales or dry lap rolls to converting facilities possessing specialized pulpers and refining equipment.
Maximum tear resistance and tensile strength remain preserved during the bleaching stage because operating temperatures stay strictly controlled.
Optical Purity
Residual impurities and short-chain hemicelluloses decrease final sheet brightness over time if chemical washing stages operate outside specified parameters. Commercial buyers measure optical cleanliness through ISO brightness meters calibrated to specific ultraviolet reflectance standards. Extended hypochlorite exposure damages cellulose chains, causing a measurable drop in folding endurance during subsequent high speed converting operations.
Papermakers monitor residual chlorine dioxide concentrations continuously to prevent yellowing during storage cycles in high humidity environments.
Tensile Yield
Refining action applied to the washed stock fibrillates individual fibres to increase interfibres bonding surface area within the wet end of the paper machine. Tensile stiffness index climbs steadily as mechanical shear forces increase the relative bonded area across the sheet structure. Converters specify targeted breaking length metrics to ensure finished carton board withstands compressive loads during pallet stacking and automated retail distribution.
Excessive mechanical treatment reduces tear strength even as tensile properties improve, requiring precise mill control over the refining intensity applied to the pulp slurry.