Analysis of breathability of linen fabrics

Breathability is the ability of a fabric to pass through air. It not only directly affects the wearing performance of the fabric, but also affects the hygienic performance of the fabric. Because the human body constantly exudes evaporated water and carbon dioxide and other substances produced by metabolism from the skin at all times, which will pollute the human body surface and the microclimate zone composed of clothing, so the fabric must have a certain degree of breathability to make the clothing inside The air should be replaced in time to keep it comfortable and clean. As a green textile that has received much attention in the new century, the breathability of linen fabric is naturally an important indicator that cannot be ignored.

Breathability

The air permeability of fabrics is often expressed by the air permeability. The air permeability refers to the volume of air flowing through the unit area of the fabric per unit time under a specified pressure difference on both sides of the fabric. The unit is L/m2.s.

Principle

By measuring the pressure difference on both sides of the flow aperture, the air permeability of the fabric is obtained. When the flow aperture is of a constant size, the greater the pressure difference, the greater the amount of air flowing per unit time; when the flow aperture is of different sizes, the same pressure difference The corresponding air flow is different. The larger the flow aperture is, the greater the air flow corresponding to the same pressure difference is.

Fabric breathability is essentially a process in which air flows from the side with higher pressure to the side with lower pressure through the fabric under the condition that there is a certain pressure difference between the air on both sides of the fabric.

There are two main ways for gas to pass through fabrics:

1. Pass through the pores of the warp and weft yarns of the fabric

2. When the airflow passes through the gaps between fibers, the interwoven pores are generally the main path. When the airflow passes through the interwoven pores, the resistance comes from viscous resistance and inertial force

Therefore, the air permeability of fabrics mainly depends on the size and number of voids in the fabric, and is related to factors such as fiber properties, fabric geometry, and finishing

Structural factors that affect fabric breathability:

Generally speaking, when the fabric structure is the same, the structural factors that affect the breathability of the fabric are mainly the total tightness of the fabric, the ratio of warp and weft tightness, and the linear density of warp and weft yarns.

The effect of total tightness on air permeability

As the total tightness increases, the yarn coverage area increases, that is, the total number of yarns in a certain area increases, causing the surface filling degree to increase and the fabric pore area to decrease, resulting in the gas channel formed between the yarns. becomes smaller, causing the viscous resistance of air flow perpendicular to the fabric to increase. Therefore, the greater the total tightness, the smaller the air permeability; but when the total tightness increases to a certain level, the rate of decrease in air permeability decreases. This is because After the total tightness increases to a certain level, the pores of the yarns in the fabric are already very small. Increasing the tightness of the fabric will only increase the squeezing of the yarns by the fabric, but the pores will change very little, and the reduction in air permeability will also be reduced. .

The effect of longitude and latitude tightness ratio on air permeability

Because as the warp and weft tightness ratio increases, the difference between the warp and weft tightness of the fabric also becomes larger and larger. For fabrics with the same tightness, when the warp and weft tightness ratio is 1, the values of the longitude and weft tightness are equal, so that The warp and weft yarns of the fabric are bound to each other, and it is difficult to slip and create gaps in both directions. Therefore, the air permeability is poor; when the warp and weft tightness ratio is 2.5, the warp tightness is much greater than the weft tightness. This makes the weft tightness very small. In this way, the weft yarns of the fabric are prone to slip and form holes, so the air permeability is large. As the warp and weft tightness ratio increases, the weft yarns slip more and more The easier it is to produce holes, that is, it is easier to form holes, so the air permeability increases at a greater and greater rate. In the same way, when the weft-to-warp tightness ratio is 1, the fabric has poor air permeability, that is, as the warp and weft tightness increases, the air permeability of the fabric increases. As the breathability ratio increases, the breathability shows an upward trend.

Effect of yarn linear density on air permeability

As the linear density of the yarn increases, the air permeability of the fabric shows an obvious upward trend, that is, the finer the yarn, the worse the air permeability of the fabric. Since the air permeability Q is inversely proportional to the square of the specific surface area (surface area per unit volume of the fiber) , and the specific surface area is inversely proportional to the diameter of the yarn, so the air permeability is proportional to the square of the diameter, that is, as the linear density of the yarn increases, the air permeability of the fabric increases.

The properties of different hemp fibers lead to:

(1) Among the three pure linen fabrics: ramie, hemp, and flax, although the ramie fabric is tighter, its air permeability is still good;

(2) The air permeability of ramie fabrics of the same specifications is much better than that of hemp fabrics. Under the same conditions, the air permeability of ramie fabrics is 189% of that of hemp fabrics;

(3) When the fabric tightness is close, the air permeability of ramie/linen interwoven fabric is significantly better than that of pure linen fabric;

(4) The air permeability of pure cotton fabric is only 1/4 ~ 1/10 of linen fabric.

The air permeability of ramie fabric is very good, not only much better than cotton fabric, but also significantly better than the other two linen fabrics. Since the selected fabrics are all natural colors, the impact of post-finishing on their air permeability can be ignored.

Analysis from the morphology of hemp fiber:
Looking at the longitudinal shape of hemp fiber

(1) Ramie is uneven in thickness, some are in the shape of round tubes, and a small amount are in the shape of flat ribbons, and are naturally twisted, like cotton fibers, but more twisted than cotton fibers.�Full; there are nodules, the cracks are inclined left and right or intersect, irregularly;

(2) Some fibers have obvious longitudinal stripes. However, flax is uniform in thickness, significantly smaller in diameter than ramie, and has a round tube shape with almost no distortion;

(3) There are obvious and clear transverse nodes, regular, and the fiber is similar to bamboo. Hemp is similar to ramie, with uneven thickness, some are round tubes, some are flat ribbons, no natural distortion, and the shape is similar to cotton. But the overall diameter is smaller than ramie;

(4) There are also transverse knots, but they are not as regular as flax, and the upper and lower uniformity of the diameter is not as good as flax.

The cross-sectional shape of hemp fiber

(1) Ramie is mostly long-waisted or round-waisted. Among the three fibers, the diameter in the length direction is larger. The cross-sections of flax and hemp are long-waisted or polygonal, and the diameter in the length direction is smaller than that of ramie. ;

(2) The cross-section of hemp fiber is quite different from that of flax. Its shapes include triangle, rectangle, waist circle, etc., and the shape is irregular

By measuring the diameters of three types of hemp fibers, it was found that;

1. Ramie has the largest average diameter, followed by hemp and flax;

2. The dispersion coefficient of fiber diameter is larger for hemp and smaller for flax. It is precisely due to the longitudinal unevenness of the ramie fiber that the gaps between the fibers in the ramie yarn increase;

3. The larger average diameter makes the specific surface area of ramie fiber smaller than the other two types of hemp fiber

According to the theory of fluid mechanics:

The air permeability is inversely proportional to the square of the specific surface area, so the air permeability of ramie fiber fabric must be higher than the other two kinds of linen fabrics. The above two reasons are superimposed, resulting in the obvious advantage of the air permeability of ramie fabric

Due to the same reasons mentioned above, the air permeability of pure cotton fabrics must be much less than that of linen fabrics.

When ramie yarn and flax yarn are used as warp and weft yarns respectively to interweave a fabric, because there are more gaps between the fibers of ramie yarn than flax yarn, the breathability of the fabric is greatly improved compared to the fabric woven with pure linen yarn.

“Conclusion”

(1) Among the three fibers of ramie, flax and hemp, due to the longitudinal unevenness and relatively small specific surface area of ramie fiber, the fabric made of ramie fiber has good air permeability.

(2) When the total tightness of the fabric is close, the air permeability of ramie/linen interwoven fabric is significantly better than that of pure linen fabric;

(3) When the total tightness of pure cotton fabric is only 9% to 20% greater than that of linen fabric, the air permeability of pure cotton fabric is only about 1/4 to 1/10 of the air permeability of linen fabric.