Breathability of Textiles

Many people refer to "moisture permeability (vapor permeability)" as "breathability". In fact, these are two completely different concepts. They are both important factors affecting physical comfort.

Definition: Breathability refers to the permeability of gas to polymer materials such as films, coatings, and fabrics. "Vapor permeability" refers to the ability of water vapor to pass through fabrics, which is also called "moisture permeability" in the industry. Only when both breathability and moisture permeability are satisfied can a comfortable body feel be obtained.

Fabric breathability refers to the ability of air to penetrate the fabric when there is a pressure difference on both sides of the fabric.

It also refers to the volume of air flowing through the unit area of the fabric per unit time under a specified pressure difference on both sides of the fabric. The commonly used unit is mm/s.

Because pressure difference is a necessary condition for air to flow, air flow can only occur in the fabric if a certain pressure difference is maintained on both sides of the fabric being tested.

Breathability affects the wearing comfort of fabrics, such as heat insulation and warmth retention. Fabric breathability is an important way for the human body to transmit heat, gaseous moisture, carbon dioxide and other gases to the outside world.

What are the testing standards for breathability?

At present, the commonly used air permeability testing methods at home and abroad mainly include GB/T 5453-1997, ISO 9237:1995, ASTM D737-2018, etc.

How to evaluate?

Foreign air permeability index requirements are usually stipulated by buyers based on the needs of product categories. There are currently no unified regulations on air permeability in my country, but there are corresponding requirements in some product standards or evaluation standards, see the table below:

Breathability requirements of relevant domestic standards:

Factors affecting breathability

Fabric factors:

1. Fiber shape and yarn structure: When the linear density of warp and weft yarns and the arrangement density of warp yarns are the same, special-shaped fiber fabrics are better than circular cross-section fiber fabrics, and thicker single fibers in the fabric have better air permeability than single thinner ones.

2. Fabric structure: Under the same arrangement density and tightness, the order of air permeability is plain weave < twill weave < satin weave < porous weave.

3. Fabric finishing processing: After fabric printing and dyeing, the fabric structure becomes tight and the breathability decreases. After the weight reduction treatment, the fabric fibers become thinner, the pores between fibers and yarns increase, and the air permeability increases.

envirnmental factor:

1. When the temperature is constant: the air permeability of fabrics shows a downward trend as the relative humidity of the air increases. After the fabric absorbs water, the fibers expand and the pores shrink, reducing the pores inside the fabric and blocking the pores in the fabric, resulting in a decrease in the breathability of the fabric.

2. When the relative humidity is constant, the air permeability of the fabric increases as the ambient temperature increases. Thermal motion of gas molecules is intensified, resulting in increased diffusion ability of molecules.

3. When the temperature and relative humidity remain unchanged, the air pressure P on both sides of the fabric changes, which will affect the measured flow rate, and the effect is nonlinear.

Summary: Breathability is an important component of fabric permeability. The air permeability of fabrics is essentially related to the size of the gaps between fibers in the fabric. Fabrics for different uses require different air permeability.

As people begin to pay more attention to the breathability of clothing in terms of comfort in wearing clothing. Depending on the season, the requirements for breathability are also different.