Long Way Education

By Eman Abdallah Kamel

Eman is a writer and textile engineer. She received her Bachelor’s degree in textile science from the Faculty of Applied Arts, Egypt.

Silk Fiber

Silk is the most essential animal fiber and the most luxurious. These natural fibers, which are highly durable and among the most expensive fabrics, are produced by the larvae of spiders and insects belonging to the phylum Arthropoda. The most useful silks are from the protein secretions of the wild Chinese Tasar moth, Antheraea pernyi, and the Indian Tasar moth, Antheraea mylitta, from the subfamily Saturniidae. In addition, the domesticated moth Bombyx mori and the subfamily Bombycinae.

Wild larvae such as the Anaphe Moloney subfamily Thaumetopoeidae produce African silk. The world’s most valuable and most important source of silk is B. mori larvae.

In the world, silk is mainly produced by China, India, Uzbekistan, Brazil, Japan, the Republic of Korea, Thailand, Vietnam, and Iran.

Kenya, Botswana, Nigeria, Zambia, Zimbabwe, Bangladesh, Colombia, Egypt, Nepal, Bulgaria, Turkey, Uganda, Malaysia, Romania, and Bolivia produce cocoons and raw silk in small quantities.

About one million workers work in the silk sector in China, 7.9 million in India, and 20,000 weaving families in Thailand.

Despite the many advantages of natural silk and its long history, the global consumption of silk is low because of its extremely high price.

History of Silk

China is the original home of silk. The earliest silk in China dates back to around 3630 BC. Henan province, the cradle of Chinese civilization, produced this silk. Chinese merchants gradually spread silk throughout Asia.

The Silk Road was some 4,000 miles long, from Eastern China to the Mediterranean Sea. The Silk Road extended northwest from the Great Wall of China, bypassing the Takla Makan desert, climbing the Pamir mountain range, crossing Afghanistan, and reaching Damascus, the capital of Syria. Afterward, the goods were transported across the Mediterranean Sea.

The silk industry emerged in Korea from the Chinese immigrants who settled there. Around 300 AD, silkworm breeding spread in India, Japan, and Persia. The silk weavers of Persia developed their patterns rather than just trying to copy Chinese styles.

During the 7th century, the Arabs conquered Persia and spread silkworms and silk fabric across Africa, Sicily, and Spain with the Arab Islamic conquests of these lands.

By the 13th century, Italian silk was very common in Europe. In the 17th century, French silk was the main rival of Italian silk.

In 1603, silk moved to America when silkworm eggs and mulberry seeds were sent to Virginia by order of King James. After that, Chinese berries were introduced from China to America to produce high-quality silk.

A silk industry flourished in Egypt around 1817, when Muhammad Ali ordered 3,000 mulberry trees to raise silkworms.

During the 19th century, the silk industry declined due to the appearance of synthetic fibers.

Life Cycle of Silkworm

Silkworms of B. mori are fed on the leaves of the mulberry tree. Tasar silkworm feeds on Arjun and oak leaves. Some other types of silk feed on pines and castor oil plant leaves. Depending on the type of egg laid, the B. mori silkworm’s life cycle may last between 55 and 60 days.

1. Eggs: Female moths lay eggs in the summer or early autumn. The size of the egg is approximately equal to the ink point size. New larvae emerge from the egg in the spring.
2. Larvae: The larval stage lasts for about 27 days. Silkworms have hair and are 1/8 of an inch long after hatching. Larvae pass through five growth stages, during which they feed on the mulberry leaves. Silkworms molt four times during the five feeding stages, or instars. During the first molting, larvae shed all their hair to gain soft skin. At the end of the fifth feeding stage, the larva molts again for about 24 hours. In the end, the silkworm is looking for some form of support on which to spin a fibrous network to hold the right cocoons.
3. Cocoons: The color of the cocoon depends on what silkworms eat, ranging from white to golden yellow. Forming a cocoon takes 3–6 days due to the synchronous extrusion of silk filaments. A viscous liquid is produced by two modified salivary glands on the larva’s head and forced out through the spinnerets. The protein gum sericin connects the two fibroin filaments that come out of the spinneret to form a single filament with a diameter of 15–25 μm. To protect the larva until it transforms into a brown pupa, sericin binds the silk threads in the cocoon together. During metamorphosis, pupae become adult moths after 15–21 days.
4. Adult moth: The moth comes out of the cocoon by secreting an enzyme that reduces the sericin, allowing it to make its way through the cocoon. The moth lives for only a few days, during which the female is fertilized. Once the eggs are laid, the female dies, and the cycle begins again.

Types of Silk

1. Mulberry Silk
2. Non-Mulberry Silk

1. Mulberry Silk

There are three types of mulberry silk cocoons: univoltine, bivoltine, and multivoltine.

The univoltine produces only one generation during the spring, and the next generation egg passes through the period of rest until the next spring.

In the case of the bivoltine, the second-generation egg does not hatch within 10–12 days and produces the second generation in the summer, but it is the third-generation egg that is in a state of hibernation and hatches the next spring, producing only two generations per year.

The multivoltine life cycle is the shortest one because of the warmer ecological conditions where it is reared, so it can produce up to seven to eight generations a year in the tropics.

2. Non-Mulberry Silk

• Tasar Silk: This silk is raised in tropical and temperate climates. The global production of this type of silk is about 95%. It is produced in China, India, and Japan. Taser cocoons are large, measuring 5 x 3 cm, oval in shape, and weighing between 7 and 14 grams. The filament length ranges from 800 m to 1500 m.
• Muga Silk: India produces this silk. The color of the cocoon is golden or light brown. A single continuous thread of about 350–400 meters makes up each cocoon. The production of Muga silk is very small, and it is used to make traditional dresses in Assam, India.
• Eri Silk (Castor Silkworm): India and some parts of Burma and Africa produce this type. The filaments of Eri are not continuous. So the Eri cocoons can be spun, not reeled.
• Coan Silk: This silk is produced from Pachypasa otus larvae, which are common in Italy, Greece, and Turkey. These silkworms spin white cocoons measuring about 8.9 cm x 7.6 cm. Currently, the production of this silk is no longer present.
• Anaphe Silk: This type of silk is produced in the southern and central countries of Africa. Silkworms spin cocoons in communes enclosed by a thin layer of silk.
• Spider Silk: It is a non-insect. The silk of spiders is soft-textured but difficult to produce because spiders cannot be raised like silkworms and do not produce as many silk threads as silkworms. The production of this silk comes from Madagascan species, including Nephila madagascarensis and Epeira.
• Mussel Silk (Sea Silk): It is another non-insect obtained from a bivalve, Pinna Nobilis, found in shallow waters along the shores of Italy and Dalmatia of the Adriatic Sea. The strong brown filament (byssus) is secreted by the mussel to anchor it to a rock. The byssus is combed and spun into silk.

Microscopic View of a Silk Fiber

Silk fibers consist of two main polymers: fibroin, the basic constituent protein of silk, and sericin, which binds silk filaments (fibroin) together. Some other substances are found in silk fiber, such as carbohydrates (1.2-1.6%), wax (0.4-0.8%), inorganic matter (0.7%), and pigments (0.2%).

Fibroin forms the inner core of the silk fiber (70–80%) of the total molecular weight. Silkworm silk yarn is composed of two fibroin filaments, each 10–14 micrometers. Fibroin fibers include two polypeptide chains: 26 kDa (light chain) and 370 kDa (heavy chain), connected by disulfide bridges, which are linked to the C-terminal parts of the H-chain.

The H-L compound binds P25-glycoprotein (30 kDa) with hydrophobic interactions, which enables the formation of micellar units, which are necessary for the transfer of fibroin through the gland cavity before spinning into fibers.

X-ray diffraction shows that fibroin consists of crystalline and non-crystalline parts, where the crystalline parts are aligned with the fiber axis. The crystalline regions are built up of H-chains, while L-chains have a small strengthening role in the fibers. H-chains are distributed among 11 hydrophilic and 12 hydrophobic domains, separated by short linkages.

X-ray diffraction analysis showed that the most suitable formulations for heavy chains are pleated β-sheets, while light chains consist of non-recurrent sequences occupying marginal positions in fibers.

Heavy chain sequences are less complex and include 2377 Gly-X (glycine carboxypeptidase) motif repeats. GX is an essential part of the β-sheet and contains protein-crystalline regions, which provide stiffness and strength to the fibers.

Sericin is a glue-like protein that surrounds the fibroin threads and binds them together. It forms 25–30% of the total molecular weight of the silk. The molecular weight of sericin varies from 10 to 400 kDa, depending on the method of extraction. The sericin portion varies along with the layers of the cocoon, which is more common in the outer layer, where the fibroin fraction is lower. It is a highly hydrophilic protein with unique properties that benefit the development of cocoons, such as antibacterial properties, oxidation resistance, ultraviolet (UV) resistance, and ease of moisture absorption and release.

Two main genes, Ser1 and Ser2, encode a 38-amino acid sequence of the molecule, which is the primary structure responsible for the mechanical strength of sericin. Sericin is the major amino acid responsible for hydrogen bonding in turns and helices. Sericin is easily degraded by heat and in an alkaline environment, which occurs during routine purification methods.

Silk Properties

1. Physical Properties

• Strength: The silk thread is very strong. This strength is due to the linear polymers and the very crystalline polymers. These factors allow the formation of more hydrogen bonds regularly. Moisture causes silk to lose its strength because a large number of hydrogen bonds dissolved by water molecules cause the silk polymer to weaken.
• Flexibility: Silk fibers are flexible fibers and can stretch from 1/7 to 1/5 of their original length before they break. Silk fabrics have moderate resistance to wrinkling.
• Water absorption: Silk is less absorbent than wool and more absorbent than cotton. Silk fibers absorb water well and dry quickly. In general, silk fabrics are comfortable in summer and warm in winter.
• Heat resistance: Peptide bonds, hydrogen bonds, and salt bonds of the silk polymer system dissolve when the temperature exceeds 1000°C.
• Electrical properties: Silk is a weak conductor of electricity and tends to form a fixed charge when handling it.
• Corrosion Resistance: Silk fabric has good corrosion resistance.
• Sunlight: The color of silk fiber changes when exposed to sunlight for a long time. The ultraviolet rays from the sun cause peptide bonds to break down, leading to the yellowing of silk. Color yellowing is also due to the oxidation of the side chains on the fiber surface.

2. Chemical Properties

• Acid effect: Silk is easily decomposed by concentrated acid because it dissolves peptide bonds. Silk fibers are less affected by diluted organic acids.
• Alkaline effect: alkaline solutions cause the swelling of silk threads. This is due to the partial separation of silk polymers by alkali molecules. Generally, silk is not sensitive to alkalis, but it can be damaged if the concentration and temperature are high.
• Oxidation: Oxidizing agents, such as hydrogen peroxide and peracids, are used in bleaching pigmented silk. Oxidation reactions occur in the tyrosine side chains, amino acid residues of the main chains, and peptide bonds.
• Bleaching: The most widely used bleaching agents are sodium perborate, salt peracids, persulfate, and hydrogen peroxide. The pH (log [H+] concentration) range between 8 and 9 was found to be effective without causing alkaline hydrolysis in silk. Inhibitors such as sodium silicate are commonly used in the bath to maintain the pH of the silk fibers and control peroxide decomposition. Insulation agents are often added to the bleach as a measure of protection against the effects of copper and iron, which can have a stimulating effect on the peroxide and lead to fiber damage.

Manufacturing Process

All cocoons are collected except the filaments that are unsuitable for reeling. And the filaments are intended for supplying the next crop of eggs.

The cocoons are stifled by sun drying, steam, or hot air to get rid of the pupa inside the cocoons.

1. Reeling of Silk

Cocoons are then sorted and reeled on any of the reeling systems, such as

• Charkha Reeling: It is a manual and powered reeling machine and is widely used in the home-based sector of the Indian reeling industry. Each charkha consists of three parts: a clay platform, a distributor, and a reel. In this method, the cocoons are cooked and reeled in the same bath. The average production of raw silk per charkha per day is about one kilogram.
• Filature Basin: In a multi-end filature basin, boilers are installed, and steam is used for cooking and reeling. In this method, some additional accessories, such as Jetta-bout, pick up the thread to increase the efficiency of the cocoon feeding, and an individual break motion is provided for each reel. The average production of filature basins per day is about 600–800 grams.
• Filature Basin: In a multi-end filature basin, boilers are installed, and steam is used for cooking and reeling. In this method, some additional accessories, such as Jetta-bout, pick up the thread to increase the efficiency of the cocoon feeding, and an individual break motion is provided for each reel. The average production of filature basins per day is about 600–800 grams.

2. Spun Silk

Spun silk is used to fill cloth filaments because it is cheaper than reeled silk. Spun silk requires more twists than reeled silk to hold all the short fibers.

After boiling the gum, the fibers are dried. They are then combed to separate, straighten, and parallel. After that, the fibers are pulled between the reels many times.

Through the scouring process, sericin can be separated from brins (silk filaments combined by silkworms). The amount of sericin ranges from 22 to 30 percent, according to the species of breeds and cocoons.

Uses of Silk

• Silk fiber is often used in the manufacture of shirts, ties, blouses, high fashion, underwear, pajamas, and robes.
• Fabrics made of silk include charmeuse, shantung, crepe de chine, dupioni, noil, tussah, taffeta, and chiffon.
• Silk is used for wall coverings, upholstery, carpets, and bedding.
• The silk fiber is used in many industries, such as parachutes, bicycle tires, artillery gunpowder bags, and non-absorbable surgical sutures.

Silk Fabric Care

1. Hand washing is recommended to clean silk fabrics.
2. Use warm water, non-alkaline soap, or baby shampoo.
3. Do not use chlorine to clean silk, as chlorine will damage the silk fabric.
4. Do not soak silk fabric for more than a few minutes.
5. During rinsing, add a few spoons of distilled white vinegar to the rinse water to neutralize alkaline effects and dissolve soap residues.
6. Do not twist the silk fabric; just press it to extract water.
7. When ironing silk, turn the silk garment inside out. Place a cloth on the silk to avoid exposing the silk fibers to direct heat. Use low-temperature settings on the iron. You can spray water on the cloth to remove the wrinkles.
8. Do not use a wooden drying rack because it can leave stains on the silk.
9. Do not use direct sunlight to dry silk clothing, as it causes the yellowing of the silk fiber.

Sources

• fao.org
• books.google.com… Mohamad, Maznah (1996). The Malayhandloom weavers: a study of the rise and decline of traditional.
• United States Department of Agriculture (USDA).

©Eman Abdallah Kamel, 2023

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