What is Cobb Sizing Test and How to Perform it?

The materials like paper, paperboard, and corrugated fiberboard are a critical component to packaging, printing, and labeling, acting as the backbone for many industries. An essential quality of all paper and board means is the ability to resist water and liquid penetration. This durability is accurately assessed using the Cobb sizing test, which is an internationally recognized standard and highly reliable process of measuring the water absorbency of paper and board under controlled conditions.

In addition to specifying the quality and resistance of the materials, this Cobb sizing test also serves as a practical means of verifying the desirable performance characteristics of paper and board materials in providing adequate protection to sensitive contents from damage done by moisture.

What is Cobb Sizing Test?

A Cobb sizing test is conducted to measure the amount of water absorbed by paper, cardboard, or corrugated fiberboard over a specified period under standardized conditions. The result, known as the Cobb value, is expressed in grams per square meter (g/m²).

• A low cobb value shows that the material is less permeable to liquids and possesses greater sizing. 
• A high cobb value means that the material will absorb more water, and this can lower its strength and printability. 

The test is especially critical for packaging boards and papers that have frequent contact with water. The excessive water absorption can lead to swelling, loss of strength, and destruction of packaged articles.

How Does Cobb Sizing Test Work?

The Cobb sizing test works by exposing a paper or board sample to water for a fixed time and then measuring how much water it absorbs. Since water penetrates the material through capillary action and fiber porosity, the test reveals how well the sample resists liquid absorption. Below are the three common methodologies of how it works –

• Water Contact: A test specimen of paper or board is exposed to water on one surface for a specified standard time.
• Absorption Process: The material absorbs water through capillary action and based on the porosity of its fibers.
• Measurement: The quantity of water absorbed per unit surface area is measured in grams per square meter (g/m²). This value indicates the degree of sizing or water resistance of the sample.

Cobb Sizing Test Procedure: Step-by-Step 

Cobb sizing test procedure includes several key steps: sample preparation and conditioning, setup of the Cobb tester, adding water and starting the test, removing excess water, reweighing the sample, and calculating the Cobb value. Here are the more detailed steps – 

Prepare the Sample

Cut a test sample large enough to span the tester's opening, usually a 100-cm² test surface. Check for obvious defects and prevent creasing or pinholes. Condition specimens and blotting papers for a minimum of 24 hours at 23 °C and 50% relative humidity to equilibrate moisture, reducing variability from ambient fluctuation effects.

Set Up the Cobb Tester

Position the sample with the test side facing the water within the Cobb ring on a resilient rubber base. Clamp tightly to avoid leakage and maintain the demarcated test area. Check the cylinder is clean and level. Pre-weigh the blotting papers and have a timer, roller, and measuring cylinder ready.

Add Water and Start the Test

Measure the desired amount of water, usually 100 mL for an area with 100 cm², and pour it quickly to wet the available surface evenly without splashing. Start the timer immediately when the pour begins. Allow the surface to stay in contact for the duration of exposure while minimizing vibration or drafts that disrupt the distribution system and allow for controlled, predictable capillary uptake.

Withdraw the Excess Solution

At the end of exposure, quickly decant the water, release the clamp, and lift the sample without pulling it up off the base. Immediately place the wet surface on conditioned blotting paper. Roll a standard roller in each direction (once in both directions), using direct‐applied pressure only to soak the surface of the sample without over‐blotting and thereby releasing absorbed moisture.

Weigh the Sample

Transfer the sample immediately to the balance to reduce loss of volatile components. Weigh to the nearest 0.01 gram as wet weight. Manipulate using edges or tweezers to prevent squeezing out water. Weigh in the same orientation and with no contact with absorbent surfaces to ensure consistency from test to test.

Compute the Cobb Value

Calculate the mass increase, W₂ - W₁, and divide by the area exposed. For a 100-cm² test area, the numerical difference is equivalent to grams per square metre absorbed. Record contact time, test face, temperature, and humidity. Repeat and average the results with comments on outliers to measure material variability and acceptance requirements.

Cobb Test in Quality Assurance

The Cobb Sizing Test is a recognized quality control test used to assess the water absorbency of paper, paperboard, and related products. The Cobb Test is determined by measuring how much water a material can absorb over a defined period of time and is expressed in square meters per gram.

The Cobb Test is important to a variety of industries such as packaging, printing, and construction, where moisture resistance can have a large impact to strength, print quality, and protection of the product. By measuring the process of absorbing water, the Cobb Test provides manufacturers and consumers important performance-related information about materials and their usability in everyday life.

Cobb Sizing Tester for Cobb Test

A Cobb Sizing Tester is an essential laboratory instrument for accurately determining the water absorption capacity of paper, cardboard, and corrugated board.  In this test, a weighted sample is exposed to a fixed quantity of water for a specified duration under controlled conditions, and the mass of water absorbed is measured by weighing the wet sample. A lower Cobb value signifies better quality and durability, while a higher value highlights greater absorbency.

To achieve dependable testing outcomes, it’s important to rely on a well-engineered instrument. For consistent, precise, and reliable results in quality testing, choose the Presto Instrument Cobb Sizing Tester, trusted by leading packaging and paper manufacturers.