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Cobb Test: The Ultimate Guide to Understanding and Applying this Paper Water Resistance Test

Coaterex TeamFebruary 12, 20258 min read
Prueba Cobb Header
If you work with paper and board, especially in the packaging sector, you have likely faced the challenge of ensuring the water resistance of your materials. Water absorption can compromise structural integrity, print quality, and ultimately, customer satisfaction. Fortunately, there is a standardized and reliable method to evaluate this property: the Cobb test. This guide is based on the TAPPI T 441 standard, the industry benchmark.

What is the Cobb Test and Why is it Important?

The Cobb test is a standardized method for determining the quantity of water absorbed by the surface of paper, board, or corrugated fiberboard in a specified time. The result is expressed in g/m². According to TAPPI T 441, it is applicable to non-bibulous materials.

  • Quality Control

    Verifies that the material meets water resistance specifications.

  • Coating Optimization

    Helps evaluate the efficacy of water-based coatings.

  • Problem Prevention

    Detects risks of deformation or loss of strength due to moisture.

  • Regulatory Compliance

    Ensures compliance in regulated sectors such as food packaging.

Implementos Prueba Cobb
Imagen 1. Implementos para la prueba Cobb

Required Materials (TAPPI T 441)

  • Cobb Test Equipment (100 cm² metal cylinder)
  • Metal Roller (10 kg, 20 cm width)
  • Blotting Paper (new for each test)
  • Precision Stopwatch
  • Precision Balance (0.001 g)
  • Distilled or Deionized Water (23 ± 1 °C)
  • Graduated Cylinder (100 ml)

How to Perform the Cobb Test Step by Step

  1. 1

    Conditioning

    Condition samples in a standard atmosphere. Cut 12.5 x 12.5 cm samples.

  2. 2

    Initial Weighing

    Weigh each dry sample with 0.01 g precision.

  3. 3

    Mounting

    Place the sample on the rubber mat and secure the metal cylinder on top.

  4. 4

    Adding Water

    Pour 100 ml of water at 23°C and start the stopwatch immediately.

  5. 5

    Exposure

    Wait for the standard time (120s for paper, 1800s for corrugated board).

  6. 6

    Water Removal

    10s before the time ends, pour out the water. Remove the cylinder.

  7. 7

    Drying

    Place the sample between two sheets of blotting paper and pass the roller once back and forth.

  8. 8

    Final Weighing

    Weigh the wet sample immediately.

  9. 9

    Calculation

    (Final Weight - Initial Weight) x 100 = Cobb (g/m²).

Resumen Prueba Cobb

Interpretation of Results

< 10 g/m²Very good resistance (e.g., Refrigerated, Plastic replacement)
10-20 g/m²Good resistance (Standard packaging and labels)
20-50 g/m²Moderate resistance (Controlled humidity)
> 50 g/m²Low resistance (Requires greater protection)

Factors Influencing Accuracy

  • Constant water temperature (23 ± 1 °C)
  • Uniform roller pressure (no extra force)
  • Blotting paper quality (always new)
  • Strictly respecting timing
  • Prior conditioning of samples
Papel Impermeabilizado

Conclusion

The Cobb test is essential for evaluating water resistance. Following the TAPPI T 441 standard ensures reliable results to optimize your sustainable packaging.

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