The main component of cotton fiber is cellulose, which is a natural polymer compound. The chemical structural formula of cellulose is composed of α glucose as the basic structural unit, and the elemental composition is 44.44% carbon, 6. 17% hydrogen and 49.39% oxygen. The polymerization degree of cotton fiber is between 6000~ 1 1000. In addition, cotton fiber also contains about 5% of other substances, which are called companion organisms. Peer has influence on spinning technology, bleaching, printing and dyeing. The surface of cotton fiber contains fatty wax, commonly known as cotton wax. Cotton wax can protect cotton fiber, which is one of the reasons why cotton fiber has good spinning performance, but it is easy to melt at high temperature. Therefore, cotton cloth is easily entangled on rollers and cots. After degreasing, the hygroscopicity of raw cotton increases, and the water absorption can reach 23~24 times of its own weight. The length of cotton fiber mainly depends on cotton varieties, growth conditions and primary processing. Usually, the average length of fine cotton is 23~33mm, and that of long-staple cotton is 33 ~ 45 mm The length of cotton fiber is closely related to spinning technology and yarn quality. Generally, the longer the length, the higher the length uniformity, the less short fibers, the thinner the spinnable yarn, the more uniform the uniformity, the higher the strength, the smoother the surface and less hairiness. The shorter the length of cotton fiber, the higher the ultimate linear density of spun yarn. Generally speaking, the spinning linear density of cotton fibers of various lengths has a limit value.

The length of cotton fiber refers to the distance between the two ends when the fiber is straightened, which is one of the important physical properties of cotton fiber. The length of cotton fiber is mainly determined by cotton varieties, growth conditions, primary processing and other factors. The length of cotton fiber is closely related to yarn quality and spinning process. Cotton fiber has long length, good uniformity, less short fiber, high yarn strength, uniform evenness, smooth surface and less hairiness.

The length of cotton fiber is uneven. The length and distribution of cotton fiber are generally expressed by main length, mass length, uniformity and short fiber rate. The length of the main body refers to the length of the fiber with the most content in cotton fiber. Mass length refers to the average length of fibers longer than the main body length, which is used to determine the roller gauge during spinning. Short fiber ratio refers to the percentage of fiber weight less than a certain length limit to the total fiber. Generally, when the short fiber ratio exceeds 15%, the yarn strength and evenness will obviously deteriorate. In addition, there are length indicators such as hand-drawn length and span length. Maturity of cotton fiber refers to the thickening degree of fiber cell wall, that is, the degree of cotton fiber growth and maturity, which is closely related to various physical properties of fiber. Normal mature cotton fiber has thick cross section, high strength, many twists, good elasticity, mercerization, high cohesion between fibers and high yarn strength. Therefore, maturity can be used as a comprehensive index of cotton fiber intrinsic quality.

The maturity of cotton fibers varies greatly. Even the same batch of cotton picked after normal cotton batting will contain mature and immature fibers. Generally speaking, fiber maturity refers to the average maturity of a batch of raw cotton.

Maturity of cotton fiber is closely related to spinning process and finished product quality. Generally speaking:

(1) Cotton fiber with high maturity can stand the blow, and it is easy to remove impurities, and it is not easy to produce neps and ropes.

⑵ Cotton fiber with high maturity has low hygroscopicity, good elasticity and low twisting efficiency.

(3) Cotton fibers with high maturity have less flying flowers and falling cotton during processing, and the finished product yield is high.

(4) Medium-mature cotton fiber has high yarn strength due to fine fiber; Cotton fibers with low maturity have low yarn strength; The cotton fiber with high maturity is thicker and the yarn strength is lower, but the cotton fiber with high maturity has better wear resistance after being processed into fabric.

5. The cotton fiber with high maturity has good color absorption and the fabric is dyed evenly. Thin-walled fibers have poor color absorption, and dark fabrics are prone to white stars, which affects the appearance.

Maturity is expressed by maturity coefficient, which refers to the calibration value corresponding to the ratio of double-layer wall thickness to outer diameter after the middle section of cotton fiber returns to circular shape. In the actual inspection, the cell wall ratio method of the central cavity is adopted, that is, the ratio of the width of the visible central cavity to the thickness of the visible side wall is used to determine it.

The maturity coefficient of normal mature upland cotton is generally 1.5~2.0, and that of low-grade cotton is below 1.4. Considering the spinning process and yarn quality, the maturity coefficient is ideal at 1.7~ 1.8. The maturity coefficient of island cotton is higher than that of upland cotton, usually around 2.0. If the temperature in the area where sea island cotton is planted is low, the maturity coefficient of sea island cotton will be obviously reduced and the maturity will be poor. Cotton fiber has weak resistance to inorganic acids. Cotton fiber is very resistant to alkali, but it will cause lateral swelling. Cotton cloth can be mercerized with dilute lye.

In addition, there are impurities and defects in cotton fiber, such as sediment, leaves and boll shells. And defects such as neps and cables. They not only affect the amount of cotton used in textile, but also affect the processing and yarn quality, so they must be inspected and strictly controlled. When observing the mature cotton fiber under the microscope, we can see that there are many spiral twists on the flat ribbon fiber, which are naturally formed during the growth of cotton fiber and are called "natural twists". Natural twist is a morphological feature of cotton fiber, which can be used to distinguish cotton from other fibers. Natural twist is generally expressed by twist per unit length (1cm) 180 degrees.

The number of turns of cotton fiber is more or less. Generally, mature and normal cotton fibers turn the most, while thin-walled fibers turn very little. Overmature fibers are rod-shaped with little turning. There are also differences in the number of revolutions of different varieties of cotton. Generally, long-staple cotton turns more, while fine-staple cotton turns less. The revolution of fine cotton is about 39~65 revolutions/cm. The bending direction of cotton fiber can change continuously along the fiber length, sometimes left-handed and sometimes right-handed, which is called the opposite direction of bending, and the number of turns is about 10~ 17 times /cm. Natural bending makes cotton fiber have good cohesion and spinnability. The more natural bending, the better the quality of cotton fiber.

The formation of bending is due to the spiral arrangement of microfibers along the fiber axis during the growth and development of cotton fibers. Before boll cracking, the fiber contained more water and did not turn; Only when the cotton fiber is dry, the microfibers arranged spirally will be bent due to internal stress. The internal structure of boiled cotton fiber determines the number, direction and position of twist. When cotton fiber is twisted for a long time, the cohesive force between fibers is large, and it is not easy to break the cotton net and coil during processing, which is beneficial to the spinning process and the quality of finished products, but too many twists will reduce the strength of cotton fiber. Cotton fibers with more reversals per unit length have lower strength, while those with fewer reversals have higher strength.