Silk Fiber Production Stages

By Eman Abdallah Kamel

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

Silk is a natural protein fiber. In this article, you will learn about the steps of the silk fiber process and tips for the final quality of silk. Image source: stock.adobe.com

Silk

Silk is a natural protein fiber made up primarily of fibroin. Silk is a protein fiber produced by a variety of arthropods, including silkworms and spiders. Insect larvae produce silk during the cocooning stage. Silkworm larvae (Bombyx mori) cocoons produce the best-known silk.

Silk fibers are characterized by their high strength and elasticity, resulting in exceptional durability. Silk fibers consist of a pyramidal structure of regularly arranged beta-sheet crystals embedded in a less frequently arranged substrate.

Did You Know?

The lustrous appearance of silk is due to its triangular prismatic structure, which causes silk fabric to refract light at different angles, thus producing a variety of colors.

Silk flowchart process.

Production Stages

Cultivation,
Rearing,
Stifling,
Sorting,
Cooking,
Deflossing,
Reeling,
Chemical Treatment,
Re Reeling,
Lacing and Skeining.

1. Cultivation

Silkworms feed on mulberry leaves. Therefore, silkworm rearing involves cultivating mulberry trees, which provide a regular supply of leaves. The silkworms are introduced via disease-free eggs (DFLs).

2. Rearing

The larvae of the silkworm moth are raised in specially designed trays in temperature- and humidity-controlled rooms and are regularly fed mulberry leaves. At a certain stage, they transform into cocoons. These cocoons are made from a single thread of a substance secreted by the pupa and wound around itself for protection—these threads, when they harden, form silk. On average, one acre of farm produces 240 kilograms of cocoons annually, starting from 100 silk fibers. Depending on whether the land is dry or irrigated, farmers can harvest the cocoons 4 to 8 times a year.

3. Stifling

This process is done by sun-drying, heating, or indirectly by exposing it to steam or hot air.

4. Sorting

Irregular, flossy, thin-ended, or damaged cocoons are rejected, while healthy ones with proper shell weight and color are selected for reeling.

5. Cooking

The cocoons are then boiled to soften the silk casing, causing the sericin fibers to swell in water with a pH between 6.8 and 7.4.

6. Deflossing

Deflossing is the process of removing the unreelable silk mass from the surface of the cocoon to expose the actual spinning tip. This is done while boiling with a bamboo brush or broom.

7. Reeling

It is the process of extracting silk threads of the desired thickness from the cocoon without any breakage. The threads from several cocoons are gathered together and tied to a spinning machine. The spinning machine then automatically unwinds the cocoons and forms a single thread of yarn. The cocoons remain submerged in boiling water throughout the process. When a cocoon is depleted, it is immediately replaced with another one to ensure the thread’s consistency.

During the twisting process and securing of silk threads, a special tool called a “croissant” is used. This tool also squeezes excess water from the threads.

The twisting process can be done in three ways.

The hand-spinning system (Charkha) is a manually operated system that produces inferior and defective cocoons. The cooking and spinning processes occur in the same vat. The Charkha spinning machine is typically made at home and used for small-scale production of silk.
The cottage basin system is superior to the charkha system in terms of production rate. In this method, cooking and reeling are done separately.
The filature system is a power-driven multi-end reeling device. Continuous reeling is possible in this device.

8. Chemical Treatment of Raw Silk

Raw silk is treated to give it a smooth and lustrous appearance while also removing sericin and other impurities. The treatments listed below are used to achieve this goal.

Degumming is the process of removing impurities from silk to restore its softness. Three methods are used to accomplish this.

Extraction with water: The silk is treated with hot water (100-130°C) for up to 4 hours, which removes sericin.
Treatment with alkali/acid: Sodium carbonate, sodium bicarbonate, or caustic soda is used for alkali treatment for 0.5 to 2 hours at a pH greater than 8.5. Acid treatment done at pH < 3.0 can eliminate sericine.
Proteolytic: Enzymatic digestion using trypsin (pH 8.0) and pepsin (pH 5.2) at temperatures between 40 and 500°C is also effective for degumming.

Silk weight: After removing sericin, there is a loss of between 22 and 25% in the weight of the silk. Therefore, to compensate for this loss, the silk is treated again with heavy metals such as iron, tin, lead, and zinc, in addition to organic compounds as tanning agents.

Did You Know?

Tannins are organic compounds, mostly glucosides, that react acidically and possess strong astringent properties. Their biological role is still debated; it is believed they may contribute to the formation of cork or dyes. Tannins are economically important because of their ability to bind to certain proteins, such as those found in animal hides, to form a strong, flexible, resistant, and insoluble material known as leather. Tannins also react with iron salts to form dark blue or greenish-black compounds, which are the basis of common inks. Their astringent nature makes them useful in medicine. Tannins are also used in oil well drilling to reduce the viscosity of the drill bit without affecting its density.

9. Re Reeling

To eliminate some problems that may occur after winding, such as thread breakage, tangling, and glue stains, the silk is first wound onto small spools and then onto standard-size hanks.

10. Lacing and Skeining

The hunk is tied at both ends with colored silk thread. It is then divided into five sections and tied with cotton thread. Lacing prevents the silk from tangling during later work. The laced hunks are then placed on a skeining machine and skeined. During this process, the hunk is twisted and folded upon itself to form several tight spiral coils. These coils are then used to make books. In each book, eight coils are arranged in horizontal rows and five in vertical rows. Each book is tied with a separate cotton band in three different places and then wrapped in tissue paper. The books are stored in bales weighing two kilograms each.

Quality of Silk

Several factors influence the final quality of silk.

Raw Material Quality: The health of the silkworms, the quality of the mulberry leaves they eat, and proper cocoon storage all have an impact on the final silk quality.
Environmental Conditions: Humidity and temperature in the reeling (spinning) facility influence machine performance and silk behavior.
Water Quality: The purity of the water used in cooking affects the solubility of sericin and the smoothness of silk thread unwinding.
Operator Skill: Human expertise is required to identify problems, make changes, and ensure consistent quality.

Sources

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