Cobb Value: Meaning, Formula, Test Procedure, and Importance in Paper Testing

 Paper and board are made of cellulose fibres, and cellulose loves water. That is useful in some products and a real problem in others. A milk carton, a corrugated box stored in a humid warehouse or a sheet that has to take offset ink all need some control over how much water the surface will absorb. The Cobb Value gives that control a number, and it is one of the most widely used measures of water absorptiveness in paper testing.

It is defined as the mass of water absorbed by a given area of paper or board in a set time, and it is expressed in grams per square metre. A low result means the sheet resists water well, usually because of good sizing or coating, while a high result means the surface drinks water quickly. The test follows ISO 535 and TAPPI T441, and the contact time is written as a subscript, such as Cobb60 for 60 seconds, with 30, 120 and 1800 seconds also in use.

For the procedure, a sample is first conditioned at standard temperature and humidity and then weighed on a balance accurate to 0.001 g. It is clamped under a metal ring that exposes a test area of 100 square centimetres, and 100 ml of water is poured in. At the end of the chosen time, the water is poured out, and the sheet is placed on blotting paper and rolled once with a 10 kg roller to remove the excess. The sample is weighed again within a few seconds. Handling speed matters because the fibres keep absorbing and evaporating once the water is removed.

The formula is simple: Cobb = (m2 − m1) × 100, where m1 is the dry mass and m2 is the wet mass, both in grams. As an example, a board that gains 0.25 g in 60 seconds has a Cobb60 result of 25 g/m². Because the ring covers exactly 100 square centimetres, multiplying by 100 converts the gain into grams per square metre. Several specimens are tested, and the top and bottom faces are checked separately, since they often differ.

Paper mills use the figure to adjust sizing chemicals and to monitor coating quality. Packaging converters rely on it for liquid cartons, corrugated board and cup stock, and printers use it to predict how ink and water will behave on the sheet.

Presto Stantest manufactures Cobb sizing testers with a precise ring and clamp, a standard roller and a sturdy base for repeatable results. Visit prestogroup.com to explore the range and get expert help with measuring Cobb Value in your own lab.