Standard Procedure
Quantitative testing protocols defined by the Technical Association of the Pulp and Paper Industry govern the liquid water absorption capacity of non-bibulous paper, paperboard, and corrugated fiberboard. Standard TAPPI T 441 specifies the Cobb test method, which measures the mass of water absorbed by one square metre of substrate during a defined exposure period under controlled hydrostatic head. The procedure determines the sizing degree and water resistance of sizing chemicals applied to paper surfaces or internal furnishes.
Common test durations include sixty seconds for sizing evaluation on printing papers and thirty minutes or longer for evaluating heavy-duty corrugated packaging boards.
Operational Protocol
Apparatus requirements include a rigid metal cylinder with an internal cross-sectional area of one hundred square centimetres, a flat metal base plate, a neoprene rubber mat, and an eight-inch wide metal roller weighing ten kilograms. A conditioned test specimen is weighed on an analytical balance to the nearest milligram and clamped securely beneath the metal cylinder. One hundred millilitres of distilled water is poured into the cylinder to initiate the timed contact period.
At the conclusion of the test duration, the liquid is poured off rapidly, the specimen is unclamped, placed between standardized blotting papers, and consolidated under one forward and backward pass of the heavy roller before final weighing.
Surface Acceptance
Absorbed water weight is calculated by subtracting initial dry mass from final mass and multiplying by one hundred to express results in grams per square metre. Low Cobb values denote highly sized, water-repellent surfaces, while elevated values indicate open, hydrophilic sheets with minimal internal or surface sizing. Corrugated box converters specify maximum Cobb values on outer kraftliners to ensure containers survive moist cold-storage environments and high-humidity ocean transit without losing edge crush strength.
TAPPI T 441 provides the standard contractual verification metric for liquid barrier performance across industrial packaging and converting supply chains.
