1. Waterproofing and moisture transmission mechanism of compact fabrics
Compact waterproof fabric is made by changing the fabric structure to achieve the purpose of waterproofing and moisture permeability. Since this type of fabric is the first waterproof and breathable fabric successfully developed, its main mechanism is: simple diffusion of water vapor between yarn gaps; capillary transmission between fiber bundles; diffusion between single fibers.
In the first mechanism, the water vapor transfer process is controlled by the water vapor pressure gradient from the inside to the outside along the fabric. The role of the second transfer mechanism becomes important when the gaps between yarns are small. For the third mechanism, it mainly involves water vapor adsorbed on the fibers on the inner surface of the fabric, diffusing through the fibers, and desorbed on the outer surface of the fabric. Tight fabrics woven from absorbent or hydrophilic fibers (such as wool, cotton, viscose, etc.) have much stronger water vapor transmission capabilities than fabrics woven from similarly structured non-absorbent or hydrophobic fibers.
The hydrostatic pressure P that the waterproof fabric can withstand can be obtained by using the equation: P=2γcosθ/r
γ is the surface tension of the wetting liquid, r is the radius of the hole, and θ is the contact angle of the wetting liquid to the hole wall.
To improve water pressure resistance, it can be achieved by reducing the size of the holes between yarns or increasing the contact angle. Increasing the contact angle is generally achieved by treating the fabric with hydrophobic treatment.
2. Development and characteristics of tight waterproof fabrics
Compact waterproof clothing mainly includes early cotton Ventile type and current ultra-fine polyester fabric.
Ventlie fabric is woven from low-twist cotton yarn in the form of Oxford cloth. This structure makes the cloth surface flat and the fabric tight but not stiff. In the dry state, water vapor generated by human body perspiration diffuses through the hydrophilic fibers in the gaps between the yarns and is capillaryly transported through the fiber bundles. When exposed to rain, the hydrophilicity of cotton fibers causes the yarns to swell, reducing the gaps between the yarns from 10 μm to 3 μm.
Today’s compact waterproof fabrics are mostly made of ultra-fine polyester or nylon fibers. In this type of fabric, the fibers and yarns are closely arranged, so that the fabric can withstand water pressure of 104 to 105 Pa without hydrophobic finishing. Capillaries are formed between fiber yarns, which can transmit water vapor well due to the capillary effect.
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