4 Test Methods for Water Permeability and Water Resistance of Fabrics

1. Factors affecting the water permeability (waterproof) of fabrics:

The water permeability of a fabric is the ability of liquid water to penetrate from one side of the fabric to the other. Generally speaking, there are 3 main factors for the waterproof and water permeability of fabrics:

1. The wettability of the fiber surface

When the fiber contact angle θ<90°, the fiber aggregate material is a water-conducting material, and the tight structure will only lead to more capillary wicking and water-conducting. When the fiber θ>90°, the fiber has waterproof characteristics, and when the fabric structure is tighter (ie, the pores are smaller), the waterproof effect is better. Therefore, a fabric is only better to discuss its water repellency or water permeability when the contact angle of the fibers is known.

2, the coating of the fabric

The surface of the fabric is coated with a continuous film layer that is impermeable and insoluble in water, which reduces the water permeability of the fabric, and the fabric is thus impermeable. It is less suitable for clothing, but can be used for tarpaulins or ponchos, etc. If a waterproof and microporous coating film is used, a coated fabric with excellent waterproofness, good water permeability and good air permeability can be formed.

3. Environment

Water repellent or coated fabrics are mostly made of non-hygroscopic fibers or coating materials, so changes in relative humidity will not affect their water resistance. The water-conducting fabrics are mostly hygroscopic fiber materials. When the relative humidity increases, the moisture absorption of the fibers is enhanced, the fibers expand and the capillary action is enhanced, so the water conductivity of the fabric is enhanced. Temperature has the same effect as humidity. Therefore, the environment has a great influence on the water-conducting fabric.

2. Water permeability and waterproof test:

Measuring the water permeability or water resistance of a fabric is to measure its water repellency or water conductivity. Different methods are used according to the actual use of the fabric, and various corresponding indicators are used to express the water permeability or water resistance of the fabric.

1. Hydrostatic pressure method

The hydrostatic method refers to the water permeability of a fabric under a certain water pressure, and it is suitable for all kinds of fabrics, including those that have been waterproofed.

The water resistance of fabrics is related to the water resistance of fibers, yarns and fabric structures, and the measured results are not the same as water spray and rain on the fabric surface. The water resistance of fabrics is measured by hydrostatic pressure method, including static pressure method and dynamic pressure method. The static pressure method is to apply hydrostatic pressure on one side of the fabric, and measure the water output under this hydrostatic pressure, the water drop time, and the hydrostatic pressure value at a certain water output. The hydrostatic pressure value can be either the height of the water column or the pressure. In the actual measurement, the water permeability per unit area and unit time (mL/cm2·h) was measured. For waterproof fabrics, measure the time it takes for water droplets to appear on the other side of the sample, or observe the number of water droplets that appear on the other side after a certain period of time.

The dynamic pressure method is to apply a water pressure P that increases at a constant speed on one side of the sample until the other side is penetrated by water and shows a certain number of water droplets. The principle of the imposed water pressure P is the same as that of the static pressure method, but P is a variable. This method is more suitable for coated fabrics or fabrics with tight structure. The hydrostatic pressure is used to reflect the waterproof performance of the fabric. The waterproof performance of the fabric with a large hydrostatic pressure is strong, and the waterproof performance of the fabric with a small hydrostatic pressure is weak. The water-conducting fabric has a strong hygroscopic ability, and it is wet when it encounters water. It has no water resistance and does not generate hydrostatic pressure.