What Is The Effect Of Fabric Permeability And Water Repellency?

The permeability and water repellency of fabrics are a pair of opposite indicators. Because of the different uses of fabrics, water repellency is sometimes used to indicate the performance of fabrics to prevent water droplets from passing through.

I. permeability and water repellency of fabrics

Fabric permeability refers to the permeability of liquid water from fabric side to the other side. Water passes through the fabric through the following ways: first, the water molecules pass through the capillary through the fibers and fibers, yarns and yarns; two, the fibers absorb moisture and make the water through the fabric; three, because of the pressure of water, the water reaches the other side through the fabric gap. Therefore, the permeability and water repellency of fabrics are related to fabric structure, hygroscopicity of fibers, waxes and oils on fiber surfaces. Fabrics used for filtering fabrics require a certain degree of water permeability.

The water repellency of fabrics is to prevent the penetration of liquid water from the fabric side to the other side. Water repellency of fabrics includes water resistance and moisture resistance of fabrics. The ability of fabrics to prevent moisture penetration is called water penetration resistance. The ability of fabrics to resist moisture is called moisture resistance. Waterproof fabrics, raincoats, tents, packaging fabrics and other fabrics require better water repellency.

Fabric permeability is essentially the liquid phase transfer of water, that is, when the water pressure difference exists on both sides of the fabric, the water passes from the high pressure side to the low pressure side. When droplets are attached to the fabric surface, the angle theta formed at the tangent of the contact point on the fabric surface is called contact angle, as shown in Figure 4. Contact angle is a function of cohesion between water molecules and adhesion between water molecules and fabric surface molecules. The larger the contact angle is, the smaller the cohesion between the water molecules and the fabric surface molecules is than the water molecules. The more difficult the water molecules are to adhere, the better the water repellency is; the worse the water repellency is.

　　 Contact angle between water droplet and fabric surface

Two. Small experiments on fabric water repellency

(1) water repellency of fabrics measured by moisture permeability method

According to the water repellent mechanism of fabrics, the contact angle formed by water droplets attached to the fabric surface indicates the ability of water repellency. Contact angle theta can be obtained by direct observation. The larger the contact angle is, the smaller the cohesion between the water molecules and the fabric surface molecules is than the water molecules. The harder the water molecules are, the better the moisture resistance is.

When the droplet forms contact with the fabric surface, if the droplet spreads quickly and penetrates into the fabric, it shows that the fabric has good moisture permeability. If the water droplets are formed, it shows that the fabric has certain water repellency. Therefore, the strength of water repellency can be evaluated by interfacial tension Sigma and contact angle theta. The figure below shows a three-phase equilibrium diagram formed at the interface between solid, liquid and gas. In the diagram, the sigma 1 is the liquid gas interface force, and the sigma 2 is the solid gas interface force. The sigma 3 is the liquid solid interface force. Theta is the contact angle (the angle between the liquid solid interface and the liquid air interface).

　　 Three phase equilibrium diagram of water droplets

The contact angle theta reflects the water repellency of the fabric when theta =0. It indicates that the liquid can expand indefinitely on the fabric and show the greatest moisture permeability when theta =180. When the liquid can only form spherical droplets on the surface of the fabric, it shows completely impermeable moisture. It's customary to use theta =90. As the boundary of moisture permeability, the theta >90. As moisture, turn theta <90. As moisture permeation, the smaller the theta, the better the moisture permeability and the worse the water repellency.

(two) water repellency of fabric measured by Pen Linfa method.

When testing the fabric's water repellency, the sample is clamped in the ring gripper and placed on the frame, so that the plane of the sample is 45 horizontal. Angle. The water at normal temperature (20 degrees) was sprayed onto the surface of the sample through the sprinkler. After spraying, remove the gripper and remove the moisture on the surface of the sample in the parallel direction of the frame and the sample. Finally, the score is compared with the standard sample. 100 is divided into no wetting, 90 is slightly moist, 80 is dripping wet, 70 is a considerable part of wetting, 50 is wet, 0 is positive and negative. The sample can also be weighed to determine the amount of water.

(three) water repellency (water permeability) of fabrics measured by hydrostatic pressure method.

Traditional water permeability testing of fabrics is usually based on hydrostatic pressure tester. It uses the method of lifting the water glass tube at a certain speed and increasing the water level, increasing the water pressure on the sample gradually. When the water droplets appear below three samples below the reflector of the specimen, the water glass tube rises immediately, and the water column height (CM) of the water glass cylinder is read out by the scale. The higher the water column is, the better the water permeability of the fabric is.

Modern fabric permeability test uses pressurized water pump to generate water pressure, and pressure sensor is used to measure the water pressure. With the increase of water pressure, when the fabric surface seepage occurs, stop pressurizing, the meter shows the water pressure value, and can print test data.

Three. What are the main factors that affect the fabric's water repellency?

The water repellency of fabrics is mainly affected by fiber properties, fabric structure and finishing process. In terms of fiber properties, the fabrics with poor moisture absorption generally have better water repellency; wax and grease on the surface of the fiber can make water droplets stick to the fabric, and the contact angle is more than 90. Thus, a certain water repellency will be generated. When these waxes and oils are removed gradually with the fabric washing several times, the contact angle will be greatly less than 90. The water repellency of fabrics is greatly reduced. In terms of fabric structure, the fabric is tight enough, moisture is not easy to pass, and has a certain water repellency. In the finishing process of fabrics, such as waterproof finishing is the main way to obtain water repellency requirements. Most of the waterproof finishing agents contain long chain aliphatic hydrocarbon compounds with little adsorptive capacity to water. After finishing the compound, the fabric surface is filled with hydrophobic molecules, so that the contact angle between the droplets and the fabric surface increases, and the water molecules are not easy to adhere, thus improving the fabric's water repellency.