Cobb test (ISO 535, Tappi T441):
|Determination of the absorbing capacity of the substrate, expressed in g/m², in relation of which the degree of sizing can be established as a percentage.
Sizing has impact on the printability and the degree in which the substrate is writeable. In addition, the Cobb value plays a role when applying glue or the bonding with another substrate. Also some applications need to be watertight (e.g. deepfreeze board)
|An area of 100 cm² of substrate is weighed. During a defined time – usually 30, 60, 120 or 300 seconds – a specific volume of distilled water is poured on to the substrate that is clamped between a rubber clad metal surface and a metal ring. After respectively 20, 45, 105 or 285 seconds the water is removed and another 15 s later the excess is blotted with a metal roller of 10 kg. Then the substrate is weighed again. The difference between the weight wet and dry is de weight of the absorbed water.
A = absorptiveness
m1 = dry mass in grams of the test piece
m2 = wet mass in grams of the test piece
F = 10 000/test area in cm²Report the average weight of absorbed water of at least 5 significant values at both front and back and the standard deviation.
|This International Standard specifies a method of determining the water absorptiveness of sized paper and board, including corrugated fibreboard, under standard conditions. It may not be suitable for paper weighing less than 50 g/m2 or embossed paper. It is not suitable for porous papers such as newsprint or unsized papers such as blotting paper or other papers having a relatively high water absorptiveness.
This method is not intended to be used for precise evaluation of the writing properties of paper although it does give a general indication of suitability for use with aqueous inks.