Silk, Silkworm Life Cycle, and How to Care for Silk Fabrics
Silk is the most important animal fiber and the most luxurious type of fibers ever. This natural fiber, which is characterized by high durability as well as one of the most expensive fabrics, is produced by larvae through a variety of spiders and insects of the Arthropoda phylum.
The most useful silks are from the protein secretions of the wild Chinese Tasar moth Antheraea pernyi, the Indian Tasar moth Antheraea mylitta from the subfamily Saturniidae, the domesticated moth Bombyxmori, and subfamily Bombycinae.
African silks are derived from the wild larvae such as the Anaphe Moloney subfamily Thaumetopoeidae.
B.mori larvae are the most valuable and most important source of silk in the world.
The main silk producing countries in the world are China, India, Uzbekistan, Brazil, Japan, the Republic of Korea, Thailand, Vietnam, and Iran. Countries producing cocoons and raw silk in small quantities are Kenya, Botswana, Nigeria, Zambia, Zimbabwe, Bangladesh, Colombia, Egypt, Nepal, Bulgaria, Turkey, Uganda, Malaysia, Romania, and Bolivia.
There are 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 the extremely high price.
A Brief History of Silk
China is the original home of silk. The earliest silk in China dates back to around 3630 BC. This silk was found in the Henan province, the cradle of Chinese civilization.
Silk gradually spread from China to reach areas throughout Asia by Chinese merchants.
The Silk Road was some 4,000 miles long running from Eastern China to the Mediterranean Sea. The Silk Road followed the Great Wall of China to the northwest, bypassing the Takla Makan desert, climbing the Pamir mountain range, crossing Afghanistan and going on to the Levant, with a major trading market in Damascus, the capital of Syria. From there, the goods were shipped across the Mediterranean Sea.
The silk industry emerged in Korea by the Chinese immigrants who settled there. Around 300 AD, silkworm breeding spread in India, Japan, and Persia. Silk weavers of Persia developed their own patterns rather than just trying to copy Chinese styles.
Granada, Andalusia, Spain.
In the 7th century, the Arabs conquered Persia and spread silkworm and silk fabric throughout 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.
Silk, North America
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.
Around 1817 Muhammad Ali ordered the planting of 3,000 mulberry trees to raise the silkworms and thus flourished the silk industry at the time in Egypt.
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 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. The life cycle of the B.mori silkworm lasts about 55-60 days but may be longer depending on the type of egg laid.
- Eggs: Female moth lays 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.
- Larvae: The larval stage lasts for about 27 days. The length of the silkworm after hatching is 1/8 of an inch and has hair. Larvae pass through five growth stages during which they feed on the mulberry leaves. During the five feeding stages, or instars, the silkworm molts four times. 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, silkworm is looking for some form of support on which to spine a fibrous network to hold the right cocoons.
- Cocoons: The color of the cocoon depends on what silkworms eat, ranging from white to golden yellow. The cocoon is formed over a period of 3-6 days by synchronous extrusion of silk filaments. Two modified salivary glands, on the larva's head, produce a viscous liquid, which is forced out through the spinnerets. The two fibroin filaments that come out from the spinneret are connected together by the protein gum sericin to form a single filament with a diameter of 15-25 μm. Sericin also binds the silk threads together in the cocoon to give protection to the larva until it turns into a brown pupa. The change from pupa to adult moth during metamorphosis takes about 15-21 days.
- Adult moth: The moth comes out of the cocoon by secreting an enzyme that reduces the sericin, allowing the moth to make its way through the cocoon. The moth only lives for a few days, during which the female is fertilized. The female dies after laying eggs, and the cycle begins again.
Types of 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 from 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 which is in a state of hibernation and hatches the next spring, producing only two generations per a 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: Tasar silkworms are raised in tropical and temperate regions. Global production of this type of silk is about 95%. It is produced in China, India, and Japan. The Taser cocoon is a very large, size 5 x 3 cm, oval shape, and weight from 7 to 14 gm. The filament length ranges from 800 m to 1500 m.
- Muga Silk: It is produced in India. The color of the cocoon is golden or light brown. Each cocoon consists of a single continuous thread of about 350-400 m. The production of Muga silk is very small and is used to make traditional dresses in Assam, India.
- Eri Silk (Castor Silkworm): It is produced in India and some parts of Burma and Africa. The filaments of Eri are not continuous. So the Eri cocoons can be spun and not reeled.
- Coan Silk: This silk is produced from Pachypasa otus larvae, which is 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. The fluff is spun into raw silk which is soft and luster.
- Spider Silk: It is a non-insect. Silk of Spider is soft-textured but difficult to produce because spiders cannot be raised like silkworms and do not produce too 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.
Chemical Structure of 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 silk fiber (70-80%) of the total molecular weight. Silkworm silk yarn is composed of two fibroin filaments, each of 10-14 micrometers.
Fibroin fibers include two polypeptide chains: 26 kDa (light chain) and 370 kDa (heavy chain) connected by disulfide bridges, which linked to the C-terminal parts of H-chain.
The H-L compound binds P25-glycoprotein (30 kDa) with hydrophobic interactions, which enables the formation of micellar units, which is 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 in 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 the sericin varies from 10 to 400 kDa, depending on the method of extraction.
The sericin portion varies along 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 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.
Properties of Silk Fiber
1- Physical Properties
- Strength: Silk thread is very strong. This strength is due to the linear polymers and the very crystalline polymer. These factors allow the formation of more hydrogen bonds on a regular basis. Silk loses its strength by moisture because a large number of hydrogen bonds dissolved by water molecules causing silk polymer weakness.
- 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 swell 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 the 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, salts peracids, persulfate, and hydrogen peroxide. The pH (log [H+] concentration) range between 8 and 9 was found to be effective without causing alkaline hydrolysis to silk. Inhibitors such as sodium silicate commonly used in the bath to maintain the pH of the silk fibers and control the 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.
All cocoons are collected except the filaments that are unsuitable for reeling as well as the filaments intended for supplying the next crop of eggs.
The cocoons are stifled by sun drying, steam or hot air to get rid the pupa inside the cocoons.
1- Reeling of Silk
Cocoons are then sorted and reeled on any way of the reeling systems such as:
Charkha Reeling: The charkha country is a manual and powered reeling machine which is widely used in the home-based sector of the Indian reeling industry. Each charkha consists of three parts: clay platform, distributor and 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.
Cottage Basin: The cocoon is cooked separately in a basin and reeled in a hot water basin connected to the reeling seat. Each basin has 6-8 ends and each filament is passed through a button to clean the wastes. Silk is reeled onto a small reel and then re-reeled to the standard skein. The average production of Silk per basin per day is about 800 grams.
Filature Basin: In multi-end filature basin, boilers are installed and steam is used for cooking and reeling. In this method, there are some additional accessories such as Jetta-bout that picks up the thread to increase the efficiency of the cocoon feeding and the individual break motion are provided for each reel. The average production of filature basin per day is about 600-800 grams.
2- Spun Silk
Spun silk is less expensive than reeled silk and is usually used to fill cloth filaments. Spun silk requires more twist than reeled silk to hold all the short fibers.
After the boiling of the gum, the fibers are dried. They are then combed to separate, straighten, and parallelize them. The fibers are then pulled between the reels many times.
Through the scouring process, sericin can be separated from brins (silk filaments combined by silkworm). The amount of sericin ranges from 22-30 percent according to species of breeds and cocoons.
Uses of Silk
Like other natural fibers, silk clothing is very comfortable. 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 chiffons.
Silk is used for wall coverings, upholstery, carpets, and bedding.
Silk fiber is used in many industries such as parachutes, bicycle tires, and artillery gunpowder bags as well as non-absorbable surgical sutures.
General Tips to Maintain Silk Fabrics
- Hand washing is recommended to clean silk fabrics.
- Use warm water, non-alkaline soap or baby shampoo.
- Do not use chlorine to clean silk as chlorine will damage the silk fabric.
- Do not soak silk fabric for more than a few minutes.
- During rinsing, add a few spoons of distilled white vinegar to the rinse water to neutralize alkaline effects and dissolve soap residues.
- Do not twist silk fabric; just press it to extract water.
- When ironing silk, turn the silk garment inside out. Place a cloth on the silk to avoid exposing the silk fibers to the direct heat. Use low-temperature settings in the iron. You can spray water on the cloth to remove the wrinkles.
- Do not use a wooden drying rack, because it can leave stains on the silk.
- Do not use direct sunlight to dry silk clothing as it causes the yellowing of silk fiber.
- United States Department of Agriculture (USDA).
- Search results | FAO | Food and Agriculture Organization of the United Nations.
- Mohamad, Maznah (1996). The Malayhandloom weavers: a study of the rise and decline of traditional. https://books.google.com.eg/books?id=5Te9LWyzQvYC&pg=PA899&redir_esc=y#v=onepage&q&f=false.
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© 2019 Eman Abdallah Kamel