The Principle of the Paper Cobb Sizing Tester

Measuring Resistance to Wetting: The Principle of the Paper Cobb Sizing Tester

For users in the paper and packaging industries concerned with the water resistance or water absorbency of paper and paperboard, the Paper Cobb Sizing Tester is an essential instrument. This tester provides a standardized method to determine the Cobb value, which quantifies the amount of water absorbed by a known area of paper or board in a specific time under defined conditions. This article explores the straightforward yet critical principle behind the operation of a Paper Cobb Sizing Tester.
 

The Paper Cobb Sizing Tester operates on the principle of measuring the increase in mass of a paper or paperboard sample after it has been exposed to a specific amount of water for a defined period under controlled conditions. The Cobb value represents the mass of water absorbed per unit area of the sample.
 

The typical Paper Cobb Sizing Tester setup involves:

Cobb Test Apparatus: This consists of a metal cylinder with a defined internal area (typically 100 cm²) and a clamping mechanism to hold the paper sample firmly against a base plate. The base plate is usually covered with a rubber mat to ensure a good seal. The dimensions of the cylinder are precisely defined in standards like ISO 535 and TAPPI T 441.

Water Delivery System: A method to accurately introduce a specific volume of water (typically 100 mL for a 100 cm² test area, resulting in a water head of 1 cm) into the cylinder in contact with the paper sample.
 

Timer: A precise timer to control the duration of water contact with the paper sample (commonly 60 seconds or 1800 seconds, depending on the standard and the paper type).

Blotting Paper: Standardized blotting paper to remove excess water from the surface of the tested sample after the specified contact time.
 

High-Precision Balance: A balance to accurately weigh the paper sample before and after the water absorption test.
 

The principle of measurement is as follows:

A paper or paperboard sample of sufficient size to cover the test area of the Cobb apparatus is conditioned according to relevant standards.

The initial mass of the paper sample (m1​, in grams) is accurately weighed.

The paper sample is clamped securely in the Cobb test apparatus, exposing a defined area (e.g., 100 cm²) to the water.
 

A specific volume of water (e.g., 100 mL) is poured into the cylinder, ensuring a consistent water head on the paper surface.

The water is allowed to remain in contact with the paper for a specified time (e.g., 60 seconds).

After the specified time, the water is poured out, and the sample is carefully removed from the apparatus.

Excess water from both surfaces of the sample is immediately and carefully removed by blotting with standardized blotting paper a specified number of times.
 

The final mass of the wet paper sample (m2​, in grams) is accurately weighed.

The Cobb value (C, in g/m²) is calculated as the difference between the final mass and the initial mass, divided by the test area (in m²):

C=Am2​−m1​​

If a test area of 100 cm² (0.01 m²) is used, the formula simplifies to:

C=(m2​−m1​)×100

The Paper Cobb Sizing Tester is an offline instrument widely used in the paper industry to assess the sizing degree of paper and board, which indicates its resistance to water penetration. A lower Cobb value indicates better water resistance.
 

While the principle is relatively simple, adherence to standardized procedures regarding sample preparation, water volume, contact time, and blotting is crucial for obtaining comparable and reliable Cobb values according to international standards.
 

In conclusion, the Paper Cobb Sizing Tester operates on the principle of measuring the mass of water absorbed by a defined area of paper or board under specific conditions over a set time. By accurately weighing the sample before and after water exposure, it provides a quantitative measure of water absorbency (the Cobb value), essential for characterizing the sizing properties of paper and ensuring its suitability for applications where water resistance is important.