What is Cobb Tester? Formula, Procedure, & Applications

In the paper and packaging industry, maintaining the proper level of water absorption is essential for ensuring product quality and strength. Excessive water absorption can weaken paper, leading to damage during handling, storage, or transportation. The Cobb tester provides a standardized method to measure the water absorptiveness of paper, board, and similar materials.

It allows manufacturers to assess moisture resistance accurately and monitor consistency throughout production. By supporting early detection of variations in absorption, the Cobb tester contributes to improved product reliability and customer satisfaction. 
 

What is Cobb Tester?

A Cobb tester is a laboratory instrument used to measure the water absorption capacity of paper, paperboard, and similar materials. It determines the Cobb value, which represents the amount of water absorbed by a sample over a specific period, commonly 60 seconds. The result is expressed in grams per square meter (g/m²), indicating how much water the material absorbs per unit area.

Purpose of Cobb Tester: The Cobb tester assists in measuring the water resistance and absorbency of products, an important consideration in uses such as packaging and printing.
 

Formula for the Cobb Test

The Cobb test formula determines the amount of water a material absorbs, typically paper or cardboard, in a given area and time.

Cobb Value (g/m²) = (Wet Weight - Dry Weight) * 100 / Sample Area (m²)

The product is given in grams of water absorbed per square meter (g/m²).

Here:

• Wet weight: Weight of the sample after being exposed to water for a given time. 
• Dry Weight: Weight of the sample before being exposed to water. 
• Sample Area: The area of the material in contact with water, typically 100 cm² (0.01 m²).
• Cobb Value: Denotes the water-absorbing capacity of the material, with larger values corresponding to higher absorbency. 
   

Cobb Tester Procedure - Step by Step 

The Cobb tester measures the water absorbency of paper and other materials. The steps involved in the process include sample preparation, Cobb tester setup, addition of water, excess water removal, and more, as discussed below:

Step 1: Sample Preparation

• Cut a round sample of the material (paper, cardboard, etc.).
• Make sure the sample is bigger than the testing area.
• Balance the sample precisely on a digital balance and measure the original weight (W1).

Step 2: Cobb Tester Setup 

• Position a cork sheet or equivalent material on the platform of the Cobb tester to avoid slippage of the sample.
• Position the weighed sample on the cork sheet.
• Hold the sample in place with the clamping device of the Cobb tester.

Step 3: Addition of Water

• Pour a predetermined quantity of water (e.g., 100 ml or 250 ml) into the test cylinder of the Cobb tester.
• Ensure the water level is suitable for the test requirements and sample size.

Step 4: Timing and Testing

• Set a timer for the time duration of the specified test (e.g., 120 seconds for paper, 300 seconds for corrugated boards).
• Let the sample stay immersed in the water for the test duration.
• Reject test samples with leakage or spillage during testing.

Step 5: Excess Water Removal 

• Drain the water from the cylinder after the test duration carefully.
• Remove the sample from the test facility.
• Roll out excess water from the sample using a hand roller (or steel roller) parallel to the flutes in case of corrugated boards.

Step 6: Final Weighing

• Weigh the wet sample (after excess water removal) on the digital balance.
• Record the final weight (W2).

Step 7: Calculation of Cobb Value

• The Cobb value, indicating water absorption per unit area, may be calculated from the formula: Cobb Value (g/m²) = (Wet Weight - Dry Weight) * 100 / Sample Area (m²).
• (10000 / Sample Area in cm²).
   

Applications of Cobb Tester

The Cobb tester is used in various industries, including packaging, paper, printing, textiles, and construction materials, particularly in quality control, research, and development. 

Here is the detailed explanation: 

Paper Industry

The Cobb Tester is extensively used in the paper manufacturing industry to test the water absorbency properties of different papers. Water absorbency is a key property that directly affects the performance of the paper in printing, writing, and coating operations. In the Cobb test, a known quantity of water is put on the surface of the paper for a given duration, and the weight gained due to water absorption is calculated.

Packaging Industry

Within the packaging sector, the Cobb Tester is widely utilized to test the water absorbency of materials like corrugated fiberboard, kraft paper, and paperboard cartons. Packaging materials during manufacturing, shipping, storing, or handling are usually exposed to water. When these materials take in too much water, the mechanical strength they possess begins to diminish, resulting in loss of structural integrity, bending, or failure to keep the packaged item safe.

Printing Industry

The finish quality of a paper is heavily dependent on how it reacts to moisture, particularly during the process of applying ink. The water absorbency of the paper surface is established through the Cobb Tester, and this is an important factor for attaining precise, clear, and high-resolution print finishes.

Construction Materials 

In the construction industry, common board products include gypsum boards, laminates, plywood, and insulation panels, which serve both interior and exterior functions. These products tend to be exposed to fluctuating environmental factors, which include high humidity, intermittent contact with water, and condensation.

Quality Control & Research and Development 

In quality control laboratories and research & development labs, Cobb testing is an essential testing method for determining the moisture absorption properties of different materials. This test is of utmost significance among industries that depend on the dimensional stability, structural integrity, or surface integrity of materials when subject to water or humidity.
 

Why Use Presto Instruments Cobb Tester

Presto Instruments is the premier choice for Cobb testing due to its precise engineering, durable construction, and compliance with international testing standards. These testers deliver accurate and consistent water absorbency results, which are vital for ensuring product quality across various industries.

Our Cobb testers feature a user-friendly design and robust build, ensuring reliable performance in both laboratory and industrial settings. They are an excellent option for maintaining product consistency and optimizing quality.