The main component of cotton fiber is cellulose, and the rest is cellulose companion. The content of cotton cellulose and its companion depends on the maturity of cotton fiber. The cellulose content of fully mature cotton fiber accounts for 94% of the total weight of cotton fiber; The content of companion organism is less.

Classification by cotton varieties

(1) fine wool cotton: also known as upland cotton. The linear density and length of the fiber are medium, the general length is 25~35mm, the linear density is about 2.12~1.56 dtex (4700~6400 metric tons), and the strength is about 4.5cn. Most of the cotton grown in China belongs to this category.

(2) long staple cotton: also known as sea island cotton. The fiber is thin and long, with a general length of more than 33mm, a linear density of about 1.54~1.18dtex (6500~8500 metric tons), and a strength of more than 4.5cn. It is of high quality and is mainly used to weave superior cotton yarn finer than 10Tex. In addition to Xinjiang long staple cotton, Egypt cotton and Sudan cotton are mainly imported in China.

In addition, there are coarse staple cotton with short fiber, which has become obsolete.

Classification by primary processing of cotton

Seed cotton is harvested from cotton, which cannot be directly used for textile processing. Primary processing must be carried out first, that is, the cottonseed in seed cotton is removed to obtain lint. The primary processing is also called embossing. After seed cotton is ginned, the percentage of lint weight in the original seed cotton weight is called lint percentage. The lint percentage is generally 30-40%. According to different primary processing methods, cotton can be divided into sawtooth cotton and roller cotton.

⑴ sawtooth cotton: the lint processed by sawtooth cotton gin is called sawtooth cotton. Sawtooth cotton contains less impurities and short fibers, and its fiber length is regular with high yield. However, the fiber length is short and there are many mill defects Most fine wool cotton uses zigzag ginning.

⑵ roller cotton: the lint produced by roller cotton machine is called roller cotton. Cot cotton has many impurities and short fibers, poor fiber length uniformity and low yield. However, the fiber length operation is small, and there are few mill defects, but there are yellow roots. Skin ginning is suitable for long staple cotton, low-grade cotton, etc.

Classification by color of raw cotton

1. White cotton: cotton that is normally mature and has normal boll opening, regardless of whether the color of raw cotton is white, milky white or light yellow, it is called white cotton. Most of the raw cotton used in cotton mills is white cotton.

2. Yellow cotton: at the late stage of cotton growth, the cotton boll died after frostbite, and the pigment on the boll shell dyed on the fiber, making the raw cotton yellow. Yellow cotton is generally low-grade cotton, which is only used in cotton mills.

3. Grey cotton: cotton fibers grown in rainy areas will be affected by more rainfall, less sunshine and low temperature during growth and development or after boll opening. Raw cotton is grey white, which is called grey cotton. Grey cotton has low strength and poor quality, which is rarely used in cotton mills.

4. Colored cotton: colored cotton refers to naturally colored cotton, which is cultivated by distant hybridization, transgenic and other biotechnology based on the original colored cotton. Natural colored cotton still retains the original soft, comfortable, breathable and other advantages of cotton fiber. The cotton fabric can reduce a little printing and dyeing process and processing costs, and can avoid environmental pollution in an appropriate amount. However, the lack of hue and color fastness are not enough, and it is still under the observation of stable heredity.

Morphological characteristics

Under the microscope, it can be found that the cotton fiber is flat and twisted in the longitudinal direction, the closed end is thin, and the end growing on the cottonseed is thick and open. The cross section of cotton fiber is composed of many concentric layers, mainly including primary layer, secondary layer and middle cavity.

⒈ primary layer: it is the outer layer of cotton fiber, that is, the primary part of fiber cells. The outer skin of the primary layer is a layer of wax and pectin, with deep filiform wrinkles on the surface. The primary layer is very thin and contains little cellulose. Cellulose has a spiral network structure in the primary layer.

2. Secondary layer: below the primary layer of cotton fiber is a thin layer of germinal cells, which are closely stacked by microfibrils. The microfibrils and fiber axis are arranged in a spiral shape with an inclination angle of 25 degrees to 30 degrees. In this layer, there are almost no gaps and holes. The secondary layer is the part formed by the deposition of cotton fiber during the thickening period, almost all of which are cellulose. Due to the daily temperature difference, most cotton fibers deposit a layer of cellulose every day, so it can form the daily cycle of cotton fibers. The deposition of cellulose in the secondary layer is not uniform, but it is in the form of bundles of small fibers, inclined to the fiber axis in a spiral shape, and turns along the length of the fiber, which is the reason for the natural bending of cotton fiber. The development of secondary layer depends on the growth conditions and maturity of cotton fiber, which can determine the main physical properties of cotton fiber.

3. Middle cavity: after the growth of cotton fiber stops, the gap left in the cell wall is called the middle cavity. For the same variety of cotton fiber, the perimeter of primary cells in the middle section is roughly the same. When the secondary cell wall is thick, the middle cavity is small; When the secondary wall is thin, the middle cavity is large. When the cotton boll is mature and shows cracking, the cross section of the cotton fiber is round, and the middle cavity is also round, and the cross section of the middle cavity is equivalent to 1/2 or 1/3 of the cross section of the fiber. When the cotton boll naturally cracks, the cross section of the cotton fiber is waist round, and the cross section of the middle cavity is also flattened, and the cross section of the flattened middle cavity is only about 10% of the total fabric of the fiber.

Composition properties

The main component of cotton fiber is cellulose, which is a natural polymer compound. The chemical structure of cellulose is composed of α - glucose as the basic structural unit, and its elemental composition is 44.44% carbon, 6.17% hydrogen and 49.39% oxygen. The polymerization degree of cotton fiber is between 6000 and 11000. In addition, cotton fiber is also attached with about 5% of other substances, which is called companion. Companion has an impact on spinning process, scouring and dyeing processing. The surface of cotton fiber contains fat wax, commonly known as cotton wax. Cotton wax has a protective effect on cotton fiber, which is one of the reasons why cotton fiber has good spinning performance, but cotton wax is easy to melt at high temperature. So cotton cloth is easy to wrap around rollers and cots. After degreasing treatment, the hygroscopicity of raw cotton increases, and the hygroscopic capacity can reach 23-24 times of its own weight.

length

The length of cotton fiber mainly depends on cotton varieties, growth conditions and primary processing. Generally, the average hand pulling length of fine wool cotton is 23~33mm, and that of long staple cotton is 33~45mm. The length of cotton fiber is closely related to spinning process and yarn quality. In general, the longer the length, the higher the length uniformity, the fewer short fibers, the finer the yarn that can be spun, the more uniform the evenness, the higher the strength, and the surface is bright and clean with less hairiness; The shorter the length of cotton fiber, the higher the ultimate linear density of spun yarn. The yarn density of cotton fibers of various lengths generally has a limit value.

Cotton fiber length refers to the distance between 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. Cotton fiber length is closely related to yarn quality and spinning process. If the cotton fiber has long length, good uniformity and less lint, the yarn strength is high, the yarn evenness is uniform, the yarn surface is bright and clean, and the hairiness is less.

The length of cotton fiber is uneven, and the length and distribution of cotton fiber are generally expressed by the main length, quality length, evenness, short fiber rate and other indicators. The main length refers to the length of the fiber with the most content in cotton fiber. Quality length refers to the average length of the fiber that is longer than the main length. It is used to determine the roller spacing in the spinning process. Short fiber percentage refers to the percentage of fiber weight whose length is shorter than a certain length limit in the total fiber. Generally, when the short fiber rate exceeds 15%, the yarn strength and evenness will be significantly worse. In addition, there are length indicators such as hand pull length and span length.

Linear density

The linear density of cotton fiber refers to the degree of fiber thickness, which is one of the important quality indicators of cotton fiber. It is closely related to the maturity and strength of cotton fiber. The linear density of cotton fiber is also one of the main factors that determine the spinning characteristics and yarn quality, and is related to the handle and luster of the fabric. If the fiber is thin, the yarn strength is high, the yarn line is dry, and the fine yarn can be spun.

Maturity

The 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 the physical properties of the fiber. The normal mature cotton fiber has thick section, high strength, many twists, good elasticity, mercerization, large cohesion between fibers and high yarn strength. Therefore, maturity can be regarded as a comprehensive indicator of the intrinsic quality of cotton fiber.

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

The maturity of cotton fiber is closely related to the spinning process and the quality of finished products

(1) cotton fibers with high maturity can withstand blows, are easy to remove impurities, and are not easy to produce neps and strands.

⑵ cotton fiber with high maturity has low moisture absorption, good elasticity and low twisting efficiency.

(3) cotton fiber with high maturity has less flying and falling cotton during processing, and the finished product yield is high.

(4) cotton fiber with medium maturity has high yarn strength due to its fine fiber; The yarn strength of cotton fiber with low maturity is not high; The cotton fiber with high maturity is thicker and the yarn strength is also low, but the cotton fiber with high maturity has better wear resistance after being processed into fabric.

(5) cotton fiber with high maturity has good color absorption and uniform dyeing. The thin-walled fiber has poor color absorption and is easy to show white stars on dark fabrics, affecting 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 is restored to circular. In the actual inspection, the ratio of the middle cavity cell wall is used to determine, that is, the ratio of the visible middle cavity width to the visible side wall thickness is used to determine.

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 sea island cotton is higher than that of upland cotton, which is usually about 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 significantly reduced and the maturity will be poor.

Strength and elasticity

The strength of cotton fiber is one of the necessary conditions for fiber to have spinning performance and use value. If the fiber strength is high, the yarn strength is also high. The strength of cotton fiber is often expressed by breaking strength and breaking length. The strength of fine wool cotton is 3.5~4.5cn, and the breaking length is 21~25km; The strength of long staple cotton is 4~6cn and the breaking length is 30km Because the strength of a single cotton fiber is quite different, the strength of the cotton bundle fiber is generally measured, and then converted into the strength index of a single fiber. The elongation at break of cotton fiber is 3%~7%, and the elasticity is poor.

Hygroscopicity

Cotton fiber is a porous material, and there are many hydrophilic genes (- OH) on its cellulose macromolecules, so its hygroscopicity is good. Under general atmospheric conditions, the moisture regain of cotton fiber can reach about 8.5%.

Acid and alkali resistance

Cotton fiber has weak resistance to inorganic acids. Cotton fiber has high resistance to alkali, but it will cause lateral expansion. The cotton cloth can be mercerized with dilute alkali solution.

In addition, there are impurities and defects in the cotton fiber, such as sediment, leaves, bell shells, etc., and defects such as neps and strands. They affect not only the amount of cotton used in textile, but also the processing and yarn quality, so they must be inspected and strictly controlled.

chemical stability

Because the main component of cotton fiber is cellulose, it is more alkali resistant but not acid resistant. Caustic soda can make cotton fiber expand violently, increase its diameter and shorten its length, resulting in strong shrinkage of cotton products. At this time, if tension is applied to limit its shrinkage, the surface of cotton products will become smooth and bright, and the dyeing performance will be greatly improved. This process is called mercerization; If it shrinks without tension, it is called alkali shrinkage. Knitted fabrics will become tight and elastic after alkali shrinkage, and have good shape retention. Acid can deteriorate the strength of cotton fiber, especially strong acid and concentrated acid should be avoided. It can be dissolved in more than 70% concentrated sulfuric acid, and concentrated hydrochloric acid and concentrated nitric acid also have a serious impact on its strength.

Natural curl

When the mature cotton fiber is observed under the microscope, it can be seen that there are many spiral distortions on the flat ribbon fiber, which are naturally formed during the growth of cotton fiber, and are called "natural twists". Natural twist is the morphological feature of cotton fiber, which can be used to distinguish cotton from other fibers. Natural twists are generally represented by the number of twists of 180 degrees per unit length (1cm).

The number of turns on a piece of cotton fiber is small. Generally, mature and normal cotton fibers have the most turns, thin-walled fibers have few turns, and over mature fibers are rod-shaped and have few turns. There are also differences in the number of turns of different varieties of cotton. Generally, long staple cotton has more turns, while fine staple cotton has less turns. The number of turns of fine wool cotton is about 39~65/cm. The bending direction of cotton fiber can change continuously along the fiber length, sometimes left-handed, sometimes right-handed, which is called the reverse of bending, and the reverse number is about 10-17 times/cm. The more natural twists, the better the quality of cotton fiber.

The formation of twist is the result of the spiral arrangement of micro fibrils along the fiber axis during the growth and development of cotton fiber. Before boll cracking, the fiber contained more moisture, and the fiber did not turn; Only when the cotton fiber dries up, the spiral micro fibrils will turn due to the effect of internal stress. The inherent structure of the boiled cotton fiber determines the number, direction and position of the twists. When the cotton fiber has more twists, the cohesion between the fibers is large, and it is not easy to produce the phenomena of broken cotton webs and rolls in the processing, which is conducive to the spinning process and product quality of the fiber, but too many reverse twists will reduce the strength of the cotton fiber. The strength of cotton fiber with more reverse times per unit length is lower, and the strength of cotton fiber with less reverse times is higher.