Cobb test

The Cobb test is a method to determine the amount of water that is absorbed, typically for paper or board. It expresses the weight of water absorbed per surface unit of the tested material over a specific time. It is often documented in g/m². Different time units may be used, and it is therefore common to mention the chosen time frame in the note of the test. For example, a note of a Cobb60, corresponds to a test of 60 seconds.

Procedure

The proceedings of the Cobb test are described in the ISO 535:

A predefined piece of material is cut. Commonly, this is 100 cm², but other sizes are also used. The weight is determined of this piece of material.

Next, the sheet is placed in a Cobb-tester. An amount of water is added that corresponds to the size of the cut-out piece. For a sheet of 100 cm², 100 mL water is used. If a different size of sheet is used, the amount of water should be adjusted properly. Depending on the chosen time frame, the material should be kept underwater for the corresponding amount of seconds.

The wet sheet is then removed and placed on a flat surface. A sheet of dry blotting paper is placed on top of the wet test sheet. Next, a roll press is used to stroke once over the entire surface. Directly afterwards, the still wet test material is weighed again. The amount of absorbed water is the weight of the wet sheet after the test minus the initial weight of the dry sheet. Express the weight of the absorbed water again over the surface of the test sample, to get the amount in g/m² This number then represents the Cobb value.

Other applications

Besides testing for water absorption, the same test setup and protocol can be used for other types of liquid. For example, a similar test can be performed with oil (SCAN P 37), as many printing colors are oil-based.