ISO 535 Cobb

Test Methodology

Clamping a circular sample under a metal ring of known area allows a controlled volume of water to be poured onto the sheet surface. After a designated exposure period, the water is quickly poured off, and excess moisture is removed with blotting paper before reweighing. Standardizing this procedure with the ISO 535 Cobb method ensures consistent results across different laboratory environments.

The resulting values, expressed in grams per square meter, guide the selection of sizing agents for boxes exposed to high humidity.

Sizing Evaluation

Internal and surface sizing adjustments are made at the paper machine based on the absorption values obtained from this test. Low values indicate a highly sized, water-resistant surface, whereas high values show an absorbent, hydrophilic sheet. Papermakers rely on the ISO 535 Cobb protocol to verify that corrugated board will resist rain and condensation during transit.