Factors affecting the breathability and moisture permeability of composite fabrics

Composite fabricBreathable and moisture-permeableability can be expressed by moisture resistance . When there is a water vapor concentration difference (or water vapor partial pressure difference) on both sides of the fabric, the resistance of moisture to pass through the fabric It is called fabric moisture resistance. It is expressed by the following formula:

R=C/q

R—-fabric fabric moisture resistance;

q——Air and moisture permeability speed (wet flow rate), kg/m2.s;

c—-Water vapor concentration difference, kg/m3.

Under stable diffusion state, The greater the moisture resistance, the smaller the moisture permeability or moisture permeability speed.

1. Temperature and humidity conditions of fiber

Experiments show that , in the fabric structure (including the volume occupied by the fabric in the fiber Ratio) under the same conditions, fiber type has almost no effect on fabric resistance. Hollis’ comparative experiments on hydrophilic-treated polyester fabrics and untreated polyester fabrics also showed that under low-humidity conditions, water vapor The transmission is not obviously related to the fiber types in the fabric. Only under commercial temperature conditions, the moisture permeability of polyester fabrics treated with hydrophilicity is significantly better than that of polyester fabrics without hydrophilic treatment. Researchers from the United States, Japan and other countries have conducted similar tests on fabrics and clothing. , the same conclusion was obtained.

Actually in low humidity conditions Since the fiber itself absorbs less moisture and the diffusion coefficient of air is larger than that of the fiber, water vapor diffuses through the pores between the fabrics to the side with lower water vapor pressure, indicating that the transmission of water vapor in the fabric has little to do with the type of fiber. 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.

Fibreboard, on the other hand The moisture absorption is also related to the temperature. During the process of soaking, the fibers must release a certain amount of heat after absorbing moisture, causing the temperature of the fiber aggregate to increase. The partial pressure of water vapor inside the fiber increases, which reduces the gradient of moisture concentration between the inside of the fiber and the outside, making the fiber The moisture absorption rate and diffusion moisture vapor transmission rate are slowed down. The diffusion coefficient of fiber will increase exponentially with the increase of temperature. This increase is more obvious when absorbing moisture. Therefore, the increase of temperature and humidity will enhance the moisture transmission ability of the fiber in the fabric. Judging from the speed of moisture absorption or moisture release, it generally starts quickly, gradually slows down as moisture absorption or moisture release increases, and finally reaches moisture absorption balance. However, the time required to reach equilibrium is related to the moisture absorption capacity of the fiber itself and the tightness of the fiber aggregate. In addition, the thermal conductivity of the fiber will increase after moisture absorption. The moisture permeability caused by the moisture absorption of the fiber itself is very complex , and there is currently no perfect theory to describe it quantitatively.

2. Fabric thickness and coverage coefficient

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 water vapor takes through the pores between the fabric. In addition, experiments show that changes in fabric porosity have a significant impact on fabric moisture resistance.

3. Fiber type and filling rate

In the case of high humidity or tight fabric structure, water vapor is no longer just transferred through the pores in the fabric but is transferred by the fiber itself. At this time, the type of fiber becomes an important factor affecting the transfer of the fabric. On the one hand, the fiber itself absorbs moisture and swells, making the fabric tighter, weakening the air permeability of the fabric, and reducing moisture transfer through pore diffusion; on the other hand, compared with the cross-sectional area of the fabric, the surface area of the fiberboard is of a considerable order of magnitude. When the moisture absorption capacity of the fiber is large, the diffusion of moisture through the fiber surface, that is, the wicking effect produced by the capillary tubes, is strengthened and becomes the main aspect of fabric moisture transmission. The reduction of fabric porosity causing the reduction of diffusion moisture permeability becomes a secondary contradiction. Therefore, as long as the fiber moisture regain in the fabric reaches a certain level, although the reduction of pores reduces the amount of moisture transfer from the air medium in the fabric, the moisture resistance of the fiber itself will increase substantially for several days, and the moisture resistance may still decrease.

Regardless of the fiber itself Whether moisture transfer or wicking moisture transfer generated by capillaries is closely related to the hydrophilicity of the fiber and the surface properties of the fiber. The test results show that under the same tightness conditions, the relationship between the moisture resistance of different types of fibers and the tightness of the fabric. Obviously, under the condition of low tightness, there is little difference in the moisture resistance of various fiber fabrics. When the density factor reaches 0.4 or higher than 0.4, the fiber surface is not smooth, the fiber cross-section is irregular, and the moisture absorption For fibers with good properties, such as cotton and wool, as the filling rate of fiber assembly increases, the moisture resistance of the fabric increases slightly, and there is a good linear relationship between the moisture resistance of the fabric and the filling rate. However, for chemical fibers such as nylon, chlorine, 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 density is greater than 0.98 The moisture resistance of g/cm3 (for glass fiber fabric) 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 hygroscopicity is significantly lower than that of non-hygroscopic fiber fabrics. In other words, the impact of fiber hydrophilicity on fabric moisture transfer is determined by the tightness of the fabric.

Therefore, the structure is relatively For loose fabrics with high porosity, when the relative humidity of the air is low, regardless of whether the fibers absorb moisture, the moisture permeability is mainly through the diffusion of gaps between fibers and yarns; and to a small extent, the moisture permeability is affected by the fibers. The influence of the type is that when the relative humidity of the air is high, the fibers with good hygroscopicity are woven into a tight fabric. After the fibers absorb moisture and expand, the gaps between the fibers are reduced, the proportion of diffusion moisture permeability is reduced, and the capillary moisture permeability in the fibers is reduced. As the proportion increases, capillary moisture permeability becomes the main factor.

4.Fabric finishing

Coating or impregnation, etc. Fabric finishing will increase the moisture resistance of the fabric. Because it increases the path of water vapor through the fabric or blocks the gaps in the fabric. However, hydrophilic finishing will increase the moisture permeability of the fabric. Water-repellent finishing generally does not affect the moisture permeability of fabrics.

5.Other factors

General fabric liquid water transmission The speed is greater than the evaporation rate of the liquid surface. There are smaller gaps and holes on the inside of the fabric, which makes it easy to condense into liquid water and transport it outwards, forming a differential capillary effect. The larger gaps and holes on the outside make it easy to meet the evaporation conditions, which is conducive to moisture dissipation. . The evaporation capacity of liquid water on the surface of the fabric is not closely related to the fabric thickness, porosity, etc., but is closely related to the uneven shape of the fabric surface, especially the size and depth of the surface pits. In general, the larger the pit opening area, The larger the radius of curvature, the higher the evaporation efficiency. The details of the pits, wind speed, temperature differences, etc. also have obvious effects.

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