Relationship Between Microstructure Of Cotton Fiber And Its Properties

　　 Cotton fibre The physical properties of cotton fiber are determined by the internal structure of the fiber, and at the same time, the physical properties determine its spinnability and use value. The physical properties of cotton fiber are naturally formed, which are closely related to the growth and development process of the fiber and the formation process of the main components. It can be said that the growth and development process of cotton fiber is not only the formation process of fiber material, but also the formation process of fiber physical properties. The physical properties of fiber are the external performance of growth and development, formation characteristics, and structural characteristics. In order to grasp the characteristics of cotton fiber, such as moisture absorption, mechanical properties, thermal properties, optical properties and the usability of cotton textiles, it is necessary to understand and grasp the growth process, formation characteristics, material composition, morphological characteristics and structural characteristics of cotton fiber. In the following, some physical properties of the fiber are analyzed from the growth and development process of cotton fiber, mainly the formation process of fiber microstructure.

1. Formation process of cotton fiber microstructure

　　 Cotton fibre It is formed by the single cell differentiation and development of cotton ovule epidermal layer. According to its formation and development process, cotton fiber differentiation and development can be divided into four stages: fiber primordial cell differentiation and protrusion stage, fiber elongation stage, fiber secondary layer thickening stage and fiber dehydration maturity stage. The differentiation of fibrous primitive cells refers to the process of differentiation of ovule epidermal cells into fibrous primitive cells. The time when fibrous cells begin to differentiate is usually difficult to determine morphologically, because a series of changes have taken place inside the cells before the changes in cell morphology. Generally, from three days before flowering to the date of flowering, fibrous primitive cells have been differentiated and formed, Under the stimulation of pollination, fiber cells continue to develop and form. After differentiation, the primitive cells that form the growth fibers expand into spherical or hemispherical processes. The early or late differentiation of the fiber cells directly affects the length of the mature fibers on the ovule. The early differentiated fibers form the growth fibers, while the fibers differentiated three days later become cotton linters. The fiber cells begin to elongate on the day of flowering, which generally takes 24 to 32 days, Among them, fiber elongation is the fastest 10 days after flowering. The fiber elongation can be divided into two stages: nonpolar expansion and polar elongation. During the nonpolar expansion, the fiber cells nonpolar expand around until the final diameter of the fiber is formed. This stage of development determines the fineness of fibers. The deposition of the secondary layer of fiber cells lasts until 40 to 50 days after flowering. The process of cellulose deposition in the cell wall, fiber elongation and thickening, is controlled by two different systems, namely, the thickening of the primary layer, but they are not completely separated. At the same time of fiber elongation, cellulose begins to deposit evenly along the entire fiber. Cellulose is deposited in the cell wall. Normally, it is one layer a day, so that the cross section of the fiber is in a layered ring, called the heliosphere.

The main component of cotton fiber is cellulose, and the rest is cellulose companion. The content of cellulose and its companion organism in cotton fiber depends on the maturity of cotton fiber. The content of cellulose in normal mature cotton fiber accounts for about 94% of the total weight of cotton fiber, and the content of companion organisms is less. The primary layer of cotton fiber is composed of 30% cellulose, neutral and acid polysaccharides, which is closely related to the surface properties of cotton fiber. The main component of the secondary layer of cotton fiber cells is cellulose, whose content is about 90% to 95%, which determines the main physical and mechanical properties of cotton fiber. Cellulose is a natural polymer, which is a polysaccharide material. Each cellulose macromolecule is formed by n glucose residue chains. The arrangement of cellulose macromolecules generally has two states, that is, some local regions are crystalline regions and some regions are amorphous amorphous regions. The fiber macromolecules in the crystallization zone are regularly and orderly arranged, relatively neat and dense, with few gaps and holes, and the binding force of each group close to each other between molecules is saturated with each other. In the same way, the areas where the macromolecules in the fiber are not regularly and orderly arranged as crystalline state are called amorphous areas, where the macromolecules are arranged in disorder and stacked loosely, where there are more gaps and holes, low density, low contact force and incomplete saturation. In one fiber, there are both crystalline and amorphous areas. The percentage of crystalline part in the whole fiber is called crystallinity. The crystallinity of cotton fiber is about 70%. When the crystallinity is high, it is difficult for the fiber to absorb moisture, with high strength and small deformation. When the crystallinity is low, it is easy to absorb moisture and dye, and shows low strength and large deformation.

At the later stage of cotton fiber growth and development, the cotton bolls split, and the fiber lost water after boll opening fibre Natural distortion occurs, which is a unique longitudinal morphological feature of cotton fiber. The amount of twist of each cotton fiber varies with the size of the spiral angle and the thickness of the secondary layer. The more mature the fiber, the thicker its cell wall, the more twist. As the main component of cotton fiber is cellulose, the deposition of cellulose in the tubular cell wall gradually stops with the extension of the growth period, and finally forms a lumen. At the same time, when the cell wall of cotton fiber is deposited, the cellulose is distributed layer by layer in the form of spiral fibrils, and the spiral direction is from left to right, so when the fiber is dry and contracted, the cell wall is twisted from left to right, forming an irregular natural twist. Therefore, the special structure of cotton fiber determines its unique physical characteristics.

Relationship between microstructure and hygroscopicity of cotton fiber

Cotton fiber is a porous material in terms of microstructure. At the same time, there are many free hydrophilic groups on the cellulose macromolecule, which can absorb water from wet air and release water to dry air. This phenomenon is called moisture absorption of cotton fiber. The hygroscopicity of cotton fiber is a complex physical characteristic, which is mainly determined by four factors.

(1) Hydrophilic group

The main component of cotton fiber is cellulose. There are a large number of hydrophilic groups on cellulose macromolecules, which have considerable affinity for water molecules. Therefore, during the growth and development of cotton fiber, the more hydrophilic groups in the molecular structure, the greater the moisture absorption capacity of cotton fiber.

(2) Molecular arrangement

The cellulose molecular chains in cotton fiber are unevenly arranged, and there are crystalline and amorphous regions. In the crystalline region, the cellulose molecular chains are orderly arranged, and water molecules are not easy to enter. In the amorphous region, cellulose molecular chains are arranged in disorder, which is a loose network structure. Most hydrophilic groups are open to water molecules, which are easy to enter. Therefore, moisture absorption of cotton fiber mainly occurs in the amorphous region. Therefore, the lower the crystallinity of cotton fiber, the stronger the moisture absorption capacity.

(3) Surface adsorption

When cotton fiber is exposed to the atmosphere, it will absorb a certain amount of water vapor and other gases on the fiber surface, which is called surface adsorption. The surface adsorption capacity is related to the specific surface volume of cotton fiber. The surface volume of cotton fiber per unit volume is called the specific surface volume of cotton fiber. The thinner the cotton fiber, the more gaps and holes in the cotton fiber, the larger the specific surface volume, and the greater the moisture absorption.

　　(4) cellulose Companion organism

In addition to cellulose, the main components of cotton fiber include a small amount of pectin, protein, pentosan, fat, wax, and some inorganic salts. Fat and wax are hydrophobic substances, which can protect cotton fibers from moisture. Pectin, protein, pentosan and inorganic salts are hydrophilic substances, which can enhance the moisture absorption of cotton fiber.

After moisture absorption, the weight of cotton fiber will increase, the density will first increase and then decrease, the strength and elongation will increase, the electrical conductivity will increase, and the fiber volume will expand. Compared with mature cotton fiber, immature cotton fiber has a larger proportion of amorphous zone, more free hydrophilic groups, thinner fiber, and larger specific surface volume, which are conducive to fiber moisture absorption. Generally, under the same conditions, immature low-grade cotton has stronger moisture absorption. Therefore, reasonable use and adjustment of the moisture absorption of cotton fiber plays a very important role in seed cotton processing, fiber performance testing and textile production.