25-Aug-2025
Presto Instruments
Paper, paperboard, and corrugated fiberboard play a crucial role in packaging, printing, and labeling, forming the backbone of many industries. One of the most vital characteristics of these materials is their ability to withstand water and liquid infiltration. This durability is rigorously assessed through the Cobb sizing test, an internationally recognized standard that meticulously measures the water absorbency of paper and board under controlled conditions.
This test not only determines the resistance and quality of materials but also underscores their reliability in protecting contents from moisture damage, which is a crucial factor in maintaining quality and integrity.
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).
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.
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 –
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 –
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.
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.
Measure the specified water volume, commonly 100 millilitres for a 100-cm² area, and pour quickly to wet the exposed surface uniformly without splashing. Start the stopwatch immediately upon pouring. Maintain contact for prescribed duration, avoiding vibration or drafts that could disturb distribution, so capillary uptake proceeds under controlled, repeatable conditions.
Decant the water rapidly at the end of exposure, release the clamp, and lift the specimen without moving it. Put the wet face on conditioned blotting paper immediately. Roll the standard roller once in each direction with stipulated pressure to dry off the surface water only, not to over-blot and thereby express absorbed moisture artefacts.
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.
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.
In quality control, the Cobb Sizing Test is a standard method used to determine the water absorption of paper, paperboard, and similar materials. This test measures the amount of water a material can absorb over a specific period, expressed in square meters per gram.
The Cobb Test is crucial to numerous sectors, including packaging, printing, and construction, where moisture resistance can play a significant role in affecting strength, quality of print, and product protection. In measuring water absorption, the Cobb Test gives manufacturers and consumers critical data on the performance of materials, their durability, and applicability to everyday use.
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.
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