Air permeability refers to the permeability of polymer materials. Such as films, coatings, and fabrics.

Permeability of fabric refers to give pressure difference across fabric to test air passing performance.

The rate of flow of air passing perpendicularly through a give area of fabric is measured at a given pressure difference across the fabric test area over a given time period. The common unit is mm/s. Because pressure difference is the necessary condition for airflow. Only when pressure difference keep, air can be flowed on the fabric.

Air permeability will affects the wear comfort of the fabric. Such as thermal insulation and warmth. Permeability of fabric an important way for the body to spread out heat, gaseous moisture and carbon dioxide.

Standards for Permeability of fabric: GB/T 5453, ISO 9237,ASTM D737, JISL1096.Machine: Automatic Air Permeability Tester.

Fabric factor and environmental factor will affect the permeability of fabric.

Fabric factor

• Fiber morphology and yarn structure: When the linear density of the warp and weft yarns and the density of warp yarn arrangement are the same, the shaped fiber fabric is superior to the circular cross-section fiber fabric, the permeability of single fiber coarser in the fabric is better than the single fiber fine.
• Fabric structure: Under the same arrangement density and tightness, strength of permeability as follows: plain weave
• Finishing and processing of fabric: fabric printing and dyeing finishing, fabric structure is tightened, then the air permeability is reduced; reduction treatment, fabric fiber is thinned, pores between fibers and yarn are increased, and gas permeability is increased.

Environmental factor

• When the temperature is constant: the air permeability of the fabric will be decreased with the increase of the relative humidity. After the fabric absorbing the moisture, the fiber will expands and the pores shrinks, which reduces the pores inside the fabric. And the pores in the fabric are clogged, will making decreases of air permeability of fabric.
• When the relative humidity is constant: the air permeability of fabric will be increased with the increases of the temperature. The thermal motion of gas molecules is strengthened, will making the increase of molecular diffusion capacity.
• When the temperature and relative humidity are constant, the air pressure P is changed on both sides of the fabric, which will affect the measured flow rate. And this influence is nonlinear.