Factors affecting the breathability and moisture permeability of composite fabrics

The breathability and moisture permeability of composite fabrics 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 passing through the fabric is called the moisture resistance of the fabric. It is expressed by the following formula: </p

R=C/q</p

R—-fabric fabric moisture resistance;</p

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

c—-water vapor concentration difference, kg/m3.</p

稳定扩散状态下，湿阻越大，透湿能力或透湿速度越小。

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1. Temperature and humidity conditions of fiber</p

Experiments show that under the same conditions of fabric structure (including the volume proportion of fabric in fiber), fiber type has almost no effect on fabric resistance. Hollis tested hydrophilic treated polyester fabric and untreated polyester fabric. Comparative experiments on polyester fabrics also show that under low humidity conditions, the transmission of water vapor is not significantly related to the type of fibers 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 �</p

In fact, under low humidity conditions, because the fiber itself absorbs less moisture, and the diffusion coefficient of air is larger than that of the fiber, a lot of water vapor diffuses through the pores between the fabrics to the side with lower water vapor pressure, indicating that the transfer of water vapor in the fabric is related to the fiber type. Doesn’t matter much �这时织物的厚度和孔隙率或织物结构是决定织物透湿的主要因素� �</p

另一方面，纤维板的吸湿还同温度有关� 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 Moisture absorption speed and diffusion moisture vapor transmission speed slow down �纤维的扩散系数会随温度的升高而呈指数增大，在吸湿时这种增加更为明显，因此温，湿度的增加会使织物内纤维的传湿能力加强� �从吸湿或放湿的速度来看，一般表现为开始较快，随吸湿或放湿的增加而逐渐减慢，终达到吸湿平衡� �但过到平衡所需时间则与纤维自身的吸湿能力和纤维集合体的松紧程度有关� �此外，吸湿后纤维的导热系数将增大� �纤维自身吸湿导致的透湿作用十分复杂,目前尚未有很完善的理论来定量描述� �</p

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2. Fabric thickness and coverage coefficient</p

织物的厚度与其湿阻有近似的。一般织物厚度越厚，织物湿阻越大。这是因为织物厚度越厚，水汽通过织物间的孔隙所走路径越长。另外，实验表明，织物孔隙率的变化对织物湿阻的影响是明显的。

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3. Types of fibers and filling rate</p

在高湿或织物结构较紧密的情况下，水汽不再只是经过织物中的孔隙传递而是由纤维自身进行传递，此时纤维的种类成为影响织物传递的重要因素。一方面纤维自身吸湿产生溶胀，使织物更加紧密，织物的透气性减弱，依靠孔隙扩散传湿作用减小；另一方面与织物的截面积相比，纤维板的表面积是一个相当大数量级的量。纤维吸湿量较大时，水分通过纤维表面扩散即毛细管产生的芯吸作用得到了加强，成为织物传湿的主要方面，织物孔隙率减小引起扩散透湿减小成为次要矛盾。因此只要织物内纤维回潮率达到一定的程度，尽管孔隙减少使得织物内由空气介质的传湿量减少，但由于纤维自身的传湿有实质几天的增加，湿阻还是有可能减小。

而无论是纤维自身传湿还是毛细管产生的芯吸传湿都与纤维的亲水性和纤维表面性能有密切的关系� �试验结果表明在相同紧密程度条件下，不同种类纤维的水汽湿润阻与织物紧密程度的关系� 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 fibers with good hygroscopicity, such as For cotton and wool, as the fiber aggregate filling rate increases, the fabric moisture resistance increases slightly, and there is a good linear relationship between the fabric moisture resistance 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 g/cm3 (for glass fiber fabric) moisture resistance will rise sharply with the increase in bulk density and filling rate (or decrease in porosity) �吸湿性好的棉、羊毛等纤维织物的湿阻明显低于非吸湿性纤维织物的湿阻，也就是说纤维亲水性对织物传湿性的影响是通过织物紧密度来决定的� �</p

Therefore, for fabrics with a loose structure and high porosity, when the relative humidity of the air is low, regardless of whether the fibers absorb moisture, the moisture permeability is mainly diffusion through the gaps between fibers and yarns; while in small processIt is mainly affected by the type of fiber. 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 diffuse moisture permeability is reduced, and the moisture content within the fibers is reduced. The proportion of capillary moisture permeability increases, and capillary moisture permeability becomes the main factor.

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4. Fabric finishing</p

涂层或浸渍等织物后整理会增加织物的湿阻。因为它增加了水汽通过织物的路径或堵塞了织物的空隙。然而，亲水整理会使织物的透湿性增加。拒水整理一般不影响织物的透湿性。

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5. Other factors</p

Generally, the liquid water transmission speed of the fabric 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 outward, forming a differential capillary effect. There are larger gaps and holes on the outside, which makes it easy to meet the evaporation conditions. , helpful for dispersing dampness 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 concave and convex 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 pit opening area. The larger the radius of curvature, the higher the evaporation efficiency �凹坑的细节、风速、温差等也有明显的影响� �</p

Source of article: Yingjie Textile