The Basics of Cotton

Most cotton is an annual plant. Sowing is usually started in April and May, and germinates after one or two weeks of sowing, and continues to grow to form cotton plants. The buds on cotton plants generally bloom in July and August, and the flowers begin to bear fruit after fertilization. The fruit is called cotton boll. The cotton boll is divided into 3 to 5 chambers, and there are 5 to 9 cotton seeds in each chamber. The cotton boll grows from small to large and matures in 45 to 60 days. At this time, the outer shell of the cotton boll becomes hard, and the cotton batting is exposed after cracking, which is called spitting. After spitting out the seed cotton, the seed cotton can be harvested, and finally the cotton fiber can be separated from the seed cotton. So, how are cotton fibers formed in this process?

Cotton fiber is formed by the elongation and thickening of cells on the epidermal wall of the ovule. A cell grows into a fiber, one end of which grows on the surface of the cotton seed, and the other end is closed. Cotton seeds are covered with cotton fibers, and there are 10,000 to 15,000 fibers on the surface of each cotton seed, which is called seed cotton. The growth of cotton fiber can be divided into three periods: elongation period, thickening period and twisting period.

1. Elongation period: After cotton blooms, the epidermal cells of the ovule begin to bulge and elongate. After the ovule is fertilized, the primary cells continue to elong. At the same time, the cell width increases and reaches a certain length. This period is called the elongation period, which is 25-30 days. . During the elongation period, the fiber mainly grows in length, while the cell wall is extremely thin, and finally an elongated thin-walled tube with a cavity is formed.
2. Thickening stage: When the primary fiber cells stretch to a certain length, they enter the thickening stage. At this time, the length of the fiber rarely increases, and the outer circumference does not change much, but the cell wall gradually thickens by depositing a layer of cellulose from the outside to the inside, and finally a cotton fiber with thinner ends and thicker middle is formed. The thickening period is 25 to 30 days.
3. Twisting period: the cotton boll cracks and spits out, the cotton fiber is in contact with the air, the moisture in the fiber evaporates, the cell wall is twisted to form an irregular spiral, which is called natural twisting, this period is called the twisting period.

Types of cotton

Classified by cotton length and fineness:

• Fine-staple cotton: It is an upland cotton species with medium fineness and length, white or milky white in color, and silky. The fine-staple cotton is of good quality, and the fiber length is 23~33 mm.
• Long-staple cotton: It is a kind of island cotton, which is native to the Atlantic coast of South America and the islands. The long-staple cotton has good quality, thin and long fibers (33~45 mm in length), and is the raw material for high-end cotton spinning products. It is suitable for planting in cotton areas with a long growth period, less rain, and sufficient sunlight. The main countries producing long-staple cotton are Egypt, the United States, Morocco and so on.
• Coarse-staple cotton: native to India, the quality of coarse-staple cotton is poor, and the fiber is thick and short, and it is now becoming obsolete.

Classified by the color of cotton:

• White cotton: Normally mature cotton with a white, milky white or light yellow color and is called white cotton. Most of the raw cotton used in cotton spinning mills is white cotton.
• Yellow cotton: In the late stage of cotton growth, the cotton bolls die due to frost or other reasons, and the pigment on the boll shell is dyed to the fiber, making the cotton fiber yellow, which is called yellow cotton. Yellow cotton is generally a low-grade cotton, and only a small amount is used in cotton spinning mills.
• Gray cotton: During the growth and development of cotton fiber or after wadding, due to more rainy days, less sunshine, and low temperature, the maturity of cotton fiber is affected, and the color of cotton fiber is gray or gray. This kind of cotton is called gray cotton. Gray cotton has low strength and poor quality, and is rarely used in cotton spinning mills.

Structure of cotton fiber

Observed through a microscope, the cross-section of mature and normal cotton fibers is irregularly elliptical with a middle cavity; immature cotton fibers are extremely flat in cross-section and have a large cavity; over-mature cotton fibers have a round cross-section with a very large cavity. Small. The longitudinal surface of the cotton fiber is irregular, and the helical twist is constantly changing along the length direction, which is called natural twist.

Performance of cotton fiber

• Length: The length of most fibers in fine-staple cotton is 23~33mm, and the length of long-staple cotton is 33~45mm. The length of fine-staple cotton is shorter than 16 mm is called short-staple, and the short-staple limit of long-staple cotton is 20 mm. The length of raw cotton is closely related to the spinning process and the quality of the yarn. Generally, the longer the length, the higher the uniformity of the length, the less the short pile, the more uniform the yarn thickness, the higher the strength, and the smooth surface.
• Fineness: The fineness of cotton fiber refers to the thickness of the fiber section, which is generally expressed by linear density, that is, expressed in grams per unit length. Generally, long-staple cotton is thinner, ranging from 1.11 to 1.43 dtex (dtex is expressed in grams of mass of 10,000 meters of fiber); fine-staple cotton is thicker, ranging from 1.43 to 2.22 dtex. The smaller the fineness of the cotton fiber, the more conducive to improving the strength of the yarn, and finer yarn can be spun.
• Maturity: The maturity of cotton fiber refers to the degree of thickening of the fiber cell wall, that is, the degree of fullness of cellulose in the cell wall. The thicker the cell wall, the smaller the cavity and the better the maturity. The maturity of cotton fiber is almost closely related to the physical properties of the fiber. Mature and normal cotton fiber has many natural twists, high strength, good elasticity, silk-like luster, good spinnability and dyeing performance.
• Chemical stability: The main component of cotton fiber is cellulose, which is more resistant to alkali but not acid. Acid can destroy cotton fiber, but under the action of alkali, it can make cotton fiber swell, twist and disappear, so that the cross section becomes round and the fiber presents silky luster. Therefore, in the dyeing and finishing of cotton fabrics, alkaline solutions are often used for processing to obtain special effects. For example, treating cotton fabrics under lye and a certain tension can significantly increase the gloss and tensile strength of cotton fabrics. The dye absorption ability is strengthened, which is the mercerizing effect of cotton fiber, which is called mercerizing.
• Moisture absorption: Cotton fiber has strong moisture absorption capacity, and the pure cotton fabric has good moisture permeability and air permeability, and it is comfortable to wear. Absorbent cotton fiber can absorb liquid water up to more than 8 times the weight of the dry fiber itself, and cotton wool is widely used in medicine and health by using this property.

Natural colored cotton

Although colored cotton has long existed in nature, it is not suitable for mechanical processing due to its relatively thick and short fiber and poor spinnability. At the same time, the color of cotton is too light, so in the modern textile industry, this textile resource has not been developed and utilized. Since the 1970s, with the increasing attention of the international community to environmental protection issues and the rapid development of biotechnology, major cotton-producing countries in the world have carried out research on colored cotton using biotechnology, and have achieved preliminary results. Among the countries researching and cultivating colored cotton, the United States is one of the countries that started early and has bred colored cotton such as light blue, pink, light yellow, and light brown.

Color cotton has excellent performance, not only has the excellent wearability of ordinary cotton, but also is an environmentally friendly fiber, which can save traditional dyeing processing, which can not only avoid the harm of chemicals to the human body, but also can save energy and water without polluting the environment. Various colored or yarn-dyed fabrics processed from natural colored cotton also have the characteristics of unique style, simple and elegant colors, and strong natural flavor. What’s more peculiar is that after many times of wearing and washing, the color of the garments sewn by it is still as beautiful as before, without fading, and after 20 to 30 machine washes, the color will gradually deepen. Slowly regain the color when it was picked. Colored cotton can be processed into various textiles. People call the textiles processed from colored cotton as environmentally friendly textiles or green textiles. The selection of this type of textile has become the fashion and trend that people wear today.

How to identify the authenticity of colored cotton?

It can be identified by making a fiber section and observing the cross section of the fiber with a microscope. The color of colored cotton fiber cross section gradually fades from the center to the edge of the cross section, while the color of dyed cotton fiber cross section is just the opposite, that is, the color of the fiber slice edge is the darkest, and the color gradually fades to the center.