An Ultimate Guide to Air Permeability of Fabric

The air permeability of fabric determines its comfort and warmth. As a garment manufacturer and laboratory developer, your understanding of fabric air permeability dictates whether the fabric you choose can achieve optimal air permeability. This article will guide you through the principles of fabric air permeability, its influencing factors, and testing methods.

Definition of Air Permeability

Fabric air permeability refers to the ability of a fabric to allow air to pass through when there is a pressure difference between its two sides. Specifically, it is the volume of air that flows through a unit area of the fabric in a unit of time under a specified pressure difference. The commonly used unit for this measurement is mm/s.

Importance of Air Permeability

The fabric needs air permeability so that a person feels comfortable wearing it. It is essential for performance wear and outdoor clothing, industrial filters, tents, sailcloths, parachutes, and various clothing materials.

Air permeability affects the microclimate formed between the human body and clothing, which directly impacts the wearer’s comfort. For example, general clothing is expected to be cool, breathable, and not stuffy. This requires a microclimate that exchanges air with the environment, facilitating the dissipation of body heat. Therefore, fabrics should have a certain degree of air permeability. In contrast, winter clothing aims to retain warmth, necessitating a more stable microclimate to reduce heat loss from the body. This requires good wind resistance in clothing, meaning lower air permeability. Hence, for apparel products, the level of air permeability is an important performance attribute that cannot be overlooked.

Influencing Factors

 

FiberFabric Factors

 

Shape and Yarn Structure

With the same line density and arrangement density of warp and weft yarns, fabrics made of non-circular cross-section fibers perform better in air permeability than those with circular cross-section fibers, and thicker individual fibers in the fabric offer better air permeability than finer ones. Under the same structural conditions, natural fiber fabrics generally have better air permeability than synthetic fiber fabrics due to the irregular cross-sectional shape of natural fibers and the uneven fineness of the fibers, resulting in more voids between fibers when spun into yarn.

Fabric Structural Design

Given the same density and tightness of arrangement, generally, the air permeability of fabrics is ranked as follows: satin weave > twill weave > plain weave. Looser, sparser, and thinner fabrics have greater air permeability than denser, heavier fabrics.

Post-treatment Processing of Fabrics

Fabric dyeing and finishing tighten the structure, reducing air permeability. Conversely, reduction treatments that thin the fibers and increase the voids between fibers and yarns enhance air permeability. For example, the same kind of greige fabric can have varied levels of looseness and surface morphology due to different post-treatment processes, affecting air permeability. Treatments like sand washing, stone washing, brushing, and water washing increase the surface fluff or yarn density, reducing air permeability. Meanwhile, post-treatment processes like water-repellent, functional finishing, coating, laminating, and composite processing further close the gaps in the fabric or even form impermeable films, causing a sharp decrease in air permeability.

Environmental Factors

 

At a Constant Temperature

The air permeability of a fabric tends to decrease with increasing relative humidity. Absorbing moisture causes fibers to swell and pores to shrink, reducing the internal voids in the fabric and leading to decreased air permeability.

At a Constant Relative Humidity

The air permeability of a fabric increases with the rise in environmental temperature. The intensified thermal motion of gas molecules leads to increased diffusion capacity.

With Constant Temperature and Relative Humidity

Changes in air pressure (P) on both sides of the fabric affect the measured flow rate, and this impact is non-linear.

Air Permeability Standards

ISO 9237    ISO 7229   ASTM D737   BS 5636   JIS L1096   GBT 5453

Air Permeability Standard Chart

Air Permeability Standard Chart

Introduction to Air Permeability Tester

After understanding the principles and influencing factors of air permeability, you need to decide how to select fabrics to achieve optimal air permeability for your clothing products. At this point, what you need is an advanced and professional Air Permeability Tester. How does this essential textile laboratory instrument help you solve the issues related to fabric air permeability? Please continue reading to find out.

How to perform an Air Permeability test

The principle of air permeability testing involves measuring the airflow volume that passes vertically through a given area of the test sample under a specified pressure difference over a certain period of time and then calculating the air permeability rate. The pressure difference is a necessary condition for airflow. Only by maintaining a certain pressure difference on both sides of the fabric being tested can airflow be generated within the fabric.

Next, let’s demonstrate using Da Rong’s Air Permeability Tester YG(B)461X.

Air Permeability Experiment Operation

Sampling: Take samples according to the procedures specified in product standards or agreements between relevant parties.

Sample Conditioning: Pre-conditioning, conditioning, and testing in standard atmospheres as per Standard requirements.

Air Permeability Equipment

Sample Platform: Equipped with a circular ventilation hole having a test area of 5m², 20m², 50m², or 100m², with an error in the test area not exceeding ±0.5%. For ventilation holes with a lower test area, an appropriate sample support net should be provided.

Clamp: Capable of securely fixing the sample in a flat manner, ensuring no air leakage around the sample edges.

Rubber Gasket: Used to prevent air leakage, fitting with the clamp.

Pressure Gauge or Manometer: Connected to the test box, able to indicate a pressure drop of 50Pa, 100Pa, 200Pa, or 500Pa across the sample, with an accuracy of at least 2%.

Air Flow Stabilization Suction Device (Fan): Enables air with standard temperature and humidity to enter the sample platform, and can generate a pressure drop of 50-500Pa through the sample.

Flowmeter, Volumetric Meter, or Measuring Orifice: Displays the flow rate of the air, in units of dm3/min (L/min), with an accuracy of no more than ±2%.

Test Conditions (Recommended Values)

Test Area:20cm
Pressure Drop: 100Pa for apparel fabrics, 200Pa for industrial fabrics

Air Permeability Test Steps

  1. Mount the sample on the sample platform, avoiding edges and folds. Apply sufficient tension for a flat, undistorted sample. To prevent air leakage, place a gasket on the low-pressure side (the side of the sample platform). If the fabric has different permeability on each side, indicate the tested side in the report.
  2. Start the exhaust fan or other devices to pass air through the sample, and adjust the flow to approach the specified pressure drop. After 1 minute or when stabilized, record the orifice and the pressure difference across it.
  3. Under the same conditions, repeat the measurement at least 10 times at different parts of the same sample.
  4. Calculate the results and present the report.

FAQs

Q: What are the relations between Air Permeability and Windproofness?

A: Air permeability means that air passes through the fabric with less difficulty, allowing for easy exchange of air on both sides of the material. The higher the air permeability rate, the better the air permeability performance. Windproofness, on the other hand, is the opposite of air permeability; it refers to the difficulty of air penetrating the fabric. The lower the air permeability rate, the better the windproof performance.

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