Factors affecting air permeability and moisture permeability

The breathability and moisture permeability of composite fabric fabric can be expressed by wet resistance. When there is difference in water vapor concentration (or partial pressure difference of water vapor) on both sides of the fabric fabric, the resistance of water passing through the fabric is called fabric wet resistance.

As follows:

R=C/q    R-- fabric fabric wet resistance;Q -- breathable and moisture permeable velocity (moisture flow rate), kg/m2.s;C -- the concentration of water vapor is poor, kg/m3. Under the stable diffusion state, the greater the moisture resistance is, the smaller the moisture permeability capacity or velocity will be.

influence factor

1. The temperature and humidity conditions of fiber in fabric structure (including in fiber volume ratio) of the fabric under the same conditions, types of fiber have little impact on resistance of fabrics, fabric, hollis on the hydrophilic treated and untreated polyester fabric of polyester fabric comparative experiments also show that under the condition of low humidity, water vapor transmission and the fabric inside the fiber kinds relation is not obvious.The moisture permeability of the hydrophilic treated polyester fabric is better than that of the non-hydrophilic treated polyester fabric only at high temperature.

In fact, under low humidity conditions, because the moisture absorption of the fiber itself is less, and the diffusion coefficient of air is much larger than that of the fiber, the water vapor diffused to the side with lower water vapor pressure through the pores between the fabrics, indicating that the water vapor transfer in the fabric has little relation with the fiber type.At this time, the thickness and porosity of the fabric or fabric structure are the main factors that determine the moisture permeability of the fabric.

On the other hand, the moisture absorption of fiberboard is also related to the temperature.

In the process of moisture absorption, a certain amount of heat should be added to the fiber after moisture absorption, so that the temperature of the fiber aggregation will be increased, the water vapor partial pressure inside the fiber will be increased, and the gradient between the moisture concentration inside the fiber and the moisture concentration outside the fiber will be reduced, so that the moisture absorption speed and the diffusion and moisture permeability speed of the fiber will be slowed down.

The diffusion coefficient of the fiber will increase exponentially with the increase of temperature, which is more obvious in moisture absorption. Therefore, the increase of temperature and humidity will strengthen the moisture transmission capacity of the fiber in the fabric.From the hygroscopic or dehumidification speed, the general performance is to start faster, with the increase of hygroscopic or dehumidification and gradually slow down, finally reach the hygroscopic balance.However, the time needed to reach the equilibrium is related to the moisture absorption capacity of the fiber itself and the tightness of the fiber assembly.In addition, the thermal conductivity of the fiber will increase after moisture absorption, and the moisture permeability caused by moisture absorption of the fiber itself is very complex, so there is no perfect theory to quantitatively describe it at present.

The thickness of the fabric is similar to its wet resistance. Generally, the thicker the fabric is, the greater the wet resistance will be.

This is because the thicker the fabric, the longer the path of water vapor through the pores between the fabrics.In addition, the effect of the change of porosity on the wet resistance of the fabric is obvious.

3. Fiber type and filling rate in the case of high humidity or tight fabric structure, water vapor is no longer transmitted through the pores in the fabric but by the fiber itself. At this time, fiber type becomes an important factor affecting the fabric transmission.

On the one hand, the moisture absorption of the fiber itself causes swelling, which makes the fabric more compact, reduces the permeability of the fabric, and reduces the moisture transfer by pore diffusion.On the other hand, compared to the cross-sectional area of the fabric, the surface area of the fiberboard is a considerable order of magnitude.When the moisture absorption capacity of the fiber is large, the water diffusion through the fiber surface, that is, the core absorption effect generated by the capillary tube, is strengthened, which becomes the main aspect of the fabric moisture transfer.

Therefore, as long as the fiber moisture regain in the fabric reaches a certain degree, the moisture resistance is likely to be reduced due to the fiber's own moisture transfer for several days, although the porosity reduction reduces the moisture transfer of the air medium in the fabric.

Whether it is the fiber itself or the core moisture absorption and transmission produced by the capillary tube are closely related to the hydrophilicity and surface properties of the fiber.The results show that the moisture resistance of different fibers is related to the density of the fabric under the same density condition.Obviously under the condition of low intensity, all kinds of fabrics wet resistance difference is not big, when the density factor of 0.4 or higher than 0.4, the fiber surface is not smooth and irregular, hygroscopicity good fiber cross section fiber, such as cotton, wool, with the increase of fiber set fill rates, the moisture resistance increase amplitude is small, the linear relation between moisture resistance and filling rate is good.

However, for chemical fibers such as polyamide fiber, chlorine fiber and glass fiber, when the filling rate is large (with small porosity and large capacity), if the filling rate is greater than 39% or the porosity is less than 61%, and the bulk density of the fabric is greater than 0.98g/cm3 (for glass fiber fabric), the wet resistance will rise sharply with the increase of bulk density and filling rate (or the decrease of porosity).The moisture resistance of cotton, wool and other fiber fabrics with good moisture absorption is obviously lower than that of non-absorbent fiber fabrics. In other words, the influence of fiber hydrophilicity on the wettability of fabrics is determined by the density of fabrics.

Therefore, for fabrics with loose structure and high voidage, moisture transmission is mainly through the diffusion of gaps between fibers and yarns, no matter whether the fibers are hygroscopic or not, in the case of low air relative humidity.However, to a small extent, under the influence of fiber types, when the air relative humidity is high, the fiber with good moisture absorption is woven into a compact fabric. After the moisture absorption and expansion of the fiber, the gap between the fibers is reduced, the proportion of diffusion and moisture permeability is reduced, and the proportion of capillary permeability in the fiber is increased, so the capillary permeability becomes the main factor.

4. Fabric finishing, such as coating or dipping, will increase the wet resistance of the fabric.Because it increases water vapor through the fabric path or blocks the fabric gap.However, a hydrophilic finish will increase the moisture permeability of the fabric.Water repellent finishes generally do not affect the moisture permeability of the fabric.

5. Other factors: generally, the liquid water transport speed of the fabric is higher than the evaporation rate of the liquid surface. The fabric has a small gap in the inner side, which makes it easy to condense into liquid water and transport outward, forming a differential capillary effect.The evaporation capacity of liquid water on the fabric surface is not closely related to the thickness and porosity of the fabric, but it is closely related to the concave-convex shape of the fabric surface, especially the size and depth of the surface pits. In general, the larger the opening area of the pits, the larger the curvature radius, and the higher the evaporation efficiency.The details of the pits, wind speed, temperature difference and so on also have obvious effects.

Fabric permeability tester is used to test the air permeability of various woven fabrics,such as knitted fabrics,woven fabrics,non-woven fabrics ...

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