Action mechanism of breathable film

When the water vapor concentration on one side of the thin film is greater than the other side of the thin film, a humidity gradient pressure difference is formed. This provides the basic conditions for gas convection, and due to the formation of convection, the humidity environment on both sides of the thin film tends to be relatively balanced.

The breathable film is used as a waterproof film, which has a barrier effect on the liquid, because of the many paths in the thin film, the zigzagging channel formed by it has a large "length to diameter ratio" (L/D) value, which can be understood as a capillary. Therefore, in the case of the same liquid (such as water) and the same pressure, as long as the liquid column height of the capillary is less than the length of the capillary, it can ensure that the liquid will not leak out.

Therefore, the water insulation of the thin film directly depends on the pore size and path length of the breathable film. This performance is usually measured by the water resistance (ΔP), when the water pressure exceeds the resistance the water pressure resistance value that the microporous film can withstand, the water will leak out.

According to the above capillary principle, the water pressure resistance of the microporous film can be replaced by capillary penetration pressure ΔP, which can be expressed as follows:

ΔP = 2σcosα/d

Among them:

ΔP means penetration pressure (Pa)

σ means surface tension (N/m)

α means contact angle (degree)

d means capillary diameter

It can be seen that different pore diameters and film thicknesses can be designed to meet the safety requirements of water pressure resistance. Obviously, the water vapor transmission rate (WVTR) requirement can also be achieved by changing the micropore diameter and micropore density of the breathable film.

For the breathable film used in personal hygiene products, the principle of improving the breathable property of the film must be under the premise of ensuring safety (ΔP value).