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Factors affecting the breathability and moisture permeability of composite fabrics

Update:25-09-2020
Summary:

The breathability and moisture permeability of composit […]

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

R=C/q

R-fabric moisture resistance;

q------Ventilation and moisture permeability (wet flow rate), kg/m2.s;

c----The difference in water vapor concentration, kg/m3.

In a stable diffusion state, the greater the moisture resistance, the lower the moisture permeability or moisture permeability.

1. Fiber temperature and humidity conditions

Experiments have shown that under the same conditions of the fabric structure (including the volume ratio of the fabric in the fiber), the fiber type has almost no effect on the fabric resistance of the fabric. Hollis has no effect on the hydrophilic treated polyester fabric and the untreated polyester fabric. Comparative experiments on polyester fabrics also show that under low humidity conditions, the relationship between the transmission of water vapor and the types of fibers in the fabric is not obvious. Only under the conditions of commercial temperature, the moisture permeability of hydrophilic treated polyester fabric is significantly better than that of polyester fabric without hydrophilic treatment. Researchers in the United States, Japan and other countries have conducted similar tests on fabrics and clothing. , Got the same conclusion.

In fact, under low humidity conditions, because the fiber itself absorbs less moisture, and the diffusion coefficient of air is much larger than that of the fiber, water vapor diffuses through the pores between the fabrics to the side with lower water vapor pressure, indicating the transmission of water vapor in the fabric and fiber types It doesn't matter much. At this time, the thickness and porosity of the fabric or the 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 temperature. In the process of listening to moisture, after the fiber absorbs moisture, a certain amount of heat must be released, which will increase the temperature of the fiber assembly and increase the partial pressure of water vapor inside the fiber. This reduces the gradient of moisture concentration between the inside of the fiber and the outside. The rate of moisture absorption and diffusion and moisture permeability slow down. The diffusion coefficient of the fiber will increase exponentially with the increase of temperature, and this increase is more obvious when it absorbs moisture. Therefore, the increase of temperature and humidity will strengthen the moisture transmission capacity of the fibers in the fabric. From the point of view of the speed of moisture absorption or release, it is generally expressed as a faster start, and gradually slows down with the increase of moisture absorption or release, and finally reaches a moisture absorption balance. But the time required to reach 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. The moisture permeability caused by the fiber's own moisture absorption is very complicated, and there is no perfect theory to quantitatively describe it.

2. Fabric thickness and coverage factor

The thickness of the fabric is similar to its moisture resistance. Generally, the thicker the fabric, the greater the moisture resistance of the fabric. This is because the thicker the fabric, the longer the path for water vapor to travel through the pores between the fabrics. In addition, the experiment shows that the change of fabric porosity has obvious influence on the moisture resistance of fabric.

3. Fiber type and filling rate

In the case of high humidity or tight fabric structure, water vapor is no longer only transmitted through the pores in the fabric but transmitted by the fiber itself. At this time, the type of fiber becomes an important factor affecting the transmission of the fabric. On the one hand, the fiber itself absorbs moisture and swells, which makes the fabric more compact, the air permeability of the fabric is weakened, and the moisture transfer effect is reduced by pore diffusion; on the other hand, compared with the cross-sectional area of ​​the fabric, the surface area of ​​the fiberboard is a considerable order of magnitude. When the fiber absorbs a large amount of moisture, the diffusion of moisture through the fiber surface, that is, the wicking effect generated by the capillary tube, is strengthened, which becomes the main aspect of fabric moisture transmission. The reduction of fabric porosity causes the decrease of diffusion and moisture permeability to become a secondary contradiction. Therefore, as long as the moisture regain of the fibers in the fabric reaches a certain level, although the reduction of pores reduces the moisture transmission of the air medium in the fabric, the moisture resistance of the fibers itself increases for a few days, and the moisture resistance may still be reduced.

Whether it is the moisture transmission of the fiber itself or the wicking moisture transmission generated by the capillary tube, it is closely related to the hydrophilicity of the fiber and the surface properties of the fiber. The test results show the relationship between the moisture resistance of different types of fibers and the tightness of the fabric under the same tightness condition. Obviously, under the condition of low compactness, the moisture resistance of various fiber fabrics is not much different. When the density factor reaches 0.4 or higher than 0.4, the fiber surface is not smooth, the fiber section is irregular, and the fiber with good moisture absorption, such as For cotton and wool, as the fiber assembly filling rate increases, the increase of the fabric moisture resistance is smaller, and the linear relationship between the fabric moisture resistance and the filling rate is good. However, for chemical fibers such as nylon, chlorinated fiber, and glass fiber, when the filling rate is large (small porosity, large capacity), such as the filling rate is greater than 39% or the porosity is less than 61%, and the fabric bulk is greater than 0.98 g/cm3 (for glass fiber fabric) moisture resistance will rise sharply with the increase of bulk density and filling rate (or decrease of porosity). The moisture resistance of fiber fabrics such as cotton and wool with good moisture absorption is significantly lower than that of non-hygroscopic fiber fabrics. That is to say, the influence of fiber hydrophilicity on the moisture transmission of fabrics is determined by the tightness of the fabric.

Therefore, for fabrics with looser structure and higher porosity, when the relative humidity of the air is low, regardless of whether the fibers absorb moisture or not, the moisture permeability mainly depends on the diffusion through the gaps between fibers and yarns; The degree is affected by the type of fiber. When the relative humidity of the air is high, the fiber with good moisture absorption is woven into a compact fabric. After the fiber absorbs moisture and expands, the gap between the fibers is reduced, and the proportion of diffusion and moisture permeability is reduced. The capillary moisture permeability ratio inside increases, and the capillary moisture permeability becomes the main factor.

4. Fabric finishing

Fabric finishing such as coating or dipping will increase the moisture resistance of the fabric. Because it increases the path of water vapor through the fabric or blocks the voids of the fabric. However, hydrophilic finishing will increase the moisture permeability of the fabric. Water-repellent finishing generally does not affect the moisture permeability of the fabric.

5. Other factors

Generally, the liquid water transmission speed of the fabric is greater than the evaporation rate of the liquid surface. The inner side of the fabric has small gaps and holes to make it easy to condense into liquid water and transport outwards, forming a differential capillary effect. There are large gaps and holes on the outside to make it easy to meet the evaporation conditions. , Conducive to dispersing dampness. 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 uneven surface of the fabric, especially the size and depth of the surface pit. In general, the larger the pit opening area, The larger the radius of curvature, the higher the evaporation efficiency. The details of the pit, wind speed, temperature difference, etc. also have obvious effects.

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