Test the Water Absorbency Strength of Papers and Boards

Water absorption is an important quality parameter for paper, paperboard and corrugated packaging. During storage and transportation, packaging materials may be exposed to humidity, condensation or accidental moisture. Excessive water absorption can reduce stiffness, weaken the board and affect the protection provided to packed products.

A Cobb Sizing Tester is used to measure the amount of water absorbed by paper, paperboard and corrugated fibreboard within a specified period. The test helps manufacturers evaluate the effectiveness of sizing, coatings and moisture-resistant treatments applied to packaging materials.

What Does the Cobb Test Measure?

The Cobb test determines the quantity of water absorbed by a defined surface area of paper or board. The result is generally expressed in grams per square metre (g/m²).

A lower Cobb value usually indicates lower water absorption, while a higher value indicates that the material absorbs more moisture. However, the acceptable Cobb value depends on the paper grade, packaging application and customer specification.

Cobb testing is useful for:

• Paper and paperboard manufacturers
• Corrugated box manufacturers
• Packaging converters
• Quality control laboratories
• Printing and coating industries

How to Perform a Cobb Sizing Test

The specimen should first be conditioned according to the applicable test procedure before testing.

1. Cut a specimen larger than the test area of the instrument.
2. Weigh the dry specimen accurately and record the initial mass as M₁.
3. Place the specimen on the tester and clamp it securely.
4. Add the specified quantity of water into the cylindrical test area.
5. Start the timer immediately and maintain contact for the required test duration.
6. Drain the water after completing the exposure period.
7. Remove the specimen and eliminate excess surface water using suitable blotting paper and a roller.
8. Weigh the specimen again and record the final mass as M₂.

Any sample showing leakage around the test area should normally be rejected and tested again.

Cobb Value Calculation

For a standard test area of 100 cm², the Cobb value is calculated as:

Cobb Value (g/m²) = (M₂ − M₁) × 100

For example:

Initial weight = 4.20 g
Final weight = 4.55 g

Cobb Value = (4.55 − 4.20) × 100 = 35 g/m²

Why Cobb Testing Matters in Packaging

Excessive moisture absorption can cause paperboard to lose rigidity, change dimensions and weaken under stacking or handling conditions. Regular Cobb testing helps manufacturers monitor material consistency, compare suppliers, evaluate coatings and detect variations in the paper manufacturing process.

The Cobb Sizing Tester from Pacorr Testing Instruments provides a practical method for evaluating the water absorptiveness of paper, paperboard and corrugated packaging materials. Reliable Cobb testing helps manufacturers select suitable materials and maintain consistent packaging quality.