Textile Materials: Fabrics (15): Permeability of Fabrics
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The permeability of the fabric reflects the fabric’s ability to conduct and transmit “particles”. The particles include gas, moisture, liquid, and even particles, electrons, etc. Permeability mainly involves air permeability, air vapor permeability, water permeability and complete permeability. Work clothes (fabrics) are between the human body and the environment, forming a local air layer microclimate, which is generally called a microclimate.
1.Breathability of fabric
The ability of gas molecules to pass through the fabric is called the breathability of the fabric, which is the basic performance in the permeability of the fabric. The air permeability of fabrics is often expressed by air permeability Bp, which refers to the amount of air passing through unit area of the fabric in unit time under the condition that both sides of the fabric maintain a certain pressure difference p, mL/(cm2• s), essentially the flow velocity of the gas.
Bp=V/(AFt)
In the formula, V is the amount of air passing through unit area in t seconds (mL); AF is the area of the fabric (cm2).
Factors affecting the breathability of fabrics:
1. Fabric structure
When the density of warp and weft spinning yarns remains unchanged and the arrangement density increases, the air permeability becomes weaker. When the tightness of the fabric remains unchanged, the air permeability of the fabric decreases as the arrangement density of warp and weft yarns increases or the linear density of spinning yarns increases. Within a certain range, the twist of the spinning yarn increases, the diameter of the spinning yarn and the tightness of the fabric decrease, and the air permeability of the fabric increases. The fabric with a larger volume fraction has poorer breathability. The air permeability of fabrics generally decreases after finishing.
2. Fiber properties and yarn structure
The moisture regain of fibers has a significant impact on breathability. For example, as the moisture regain of wool fabrics increases, the air permeability decreases significantly. Most special-shaped fiber fabrics have better air permeability than circular cross-section fiber fabrics. The shorter the fiber, the greater the rigidity, the greater the probability of product hairiness, and the more obstructions and channel changes are formed, so the air permeability is worse. The tighter the structure of the spinning yarn, the smaller the transparency within the spinning yarn, and the greater the transparency between the spinning yarns. The twist and cleanness of the yarn make sense for transparency.
3. Environmental conditions
When the temperature remains constant, the air permeability of fabrics shows a downward trend as the relative humidity of the air increases. When the relative humidity is constant, the air permeability of the fabric increases as the ambient temperature increases.
2. Breathability and vapor permeability of fabric
Refers to the performance of moisture vapor permeability through fabrics, also known as water vapor permeability, referred to as air permeability or steam permeability. It is the thermal balance ability of maintaining the body’s heat production and heat dissipation when the human body dissipates heat and sweats.
The thicker the fabric, the longer the fiber transmission channel, and the more contact points between fibers, the greater the obstruction to the transmission of water molecules through the channel. The greater the fabric arrangement density and tightness, the greater the pressure between warp and weft yarns; the greater the twist of the spinning yarn, the more action points between fibers, and the better the contact. These are all conducive to the conduction of moisture in and between fibers, but the continuity and transmission speed of this conduction are much smaller than the vapor phase transmission of pores, so the density increases and the breathability of the fabric still decreases significantly.
When the relative humidity of the air increases, the evaporative heat dissipation resistance of the fabric to the human body increases, and the breathability of the fabric decreases. Increased ambient air velocity is beneficial to heat and moisture transfer in workwear (fabric).
3. Water permeability of fabric
The ability of liquid water to penetrate from one side of the fabric to the other is called the water permeability of the fabric.
When the contact angle of the fiber is less than 90°, the fiber aggregate material is a water-conducting material, and a tight structure will only lead to more capillaries and wick water.
The water permeability of the fabric can be reduced by applying a continuous film layer that is impermeable and insoluble in water on the surface of the fabric.
The increase in temperature will increase the expansion of the fiber. Since the fiber is an anisotropic material, the radial expansion is greater than the longitudinal expansion, so the result is a reduction of holes, which is beneficial to improving the water-repellent performance.
4. Transparency of fabric
Refers to the performance of the thread passing through the fabric, involving the penetration strength Id directly through the fabric pores and the penetration strength I through the fiber (including transmission, reflection and scattering) sub>f. The total strength of It is Id+If.
The permeability of the fabric first depends on the permeability of the straight channel. The lower the arrangement density of the fabric, the finer the diameter of the spinning yarn, the less hairiness, the more holes the fabric has, the greater the permeability of the fabric. Secondly, it depends on the transmission properties of the fiber. The lower the transmittance of the fiber itself, the weaker the transparency of the fabric. Different compositions of fiber materials have different absorption, reflection, and scattering properties. The higher the absorption, reflection, and scattering coefficients of fibers, the weaker the transparency of the fiber aggregate.
Fabric Materials – Fabrics (15): Permeability of Fabrics
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