Different Methods of Degumming of Silk

Degumming of Silk

Pallavi Sunil Gudulkar
Department of Textiles (Textile Chemistry)
DKTE’S Textile & Engineering Institute, Ichalkaranji, India
Intern at Textile Learner
Email: pallavigudulkar@gmail.com


1. Introduction:
Silk, a protein fiber, consists of two elements, namely fibroin, a filamentous protein, and sericin, a non-filamentous protein. The process of eliminating sericin, otherwise known as gum, is known as degumming. Degumming of silk involves the hydrolytic or enzymatic breaking of sericin’s peptide bonds, followed by the removal of sericin from the silk fibroin. Sericin can be hydrolysed in neutral, alkaline, or acidic conditions to produce four fractions with distinct characteristics. The most common degumming procedure is to boil off in an alkaline soap solution. There are numerous qualitative ways for determining the degree of degumming, but no quantitative approaches have yet been discovered. Degumming can be done on yarn or fabric, and it can be done manually or automatically. Degumming can be accomplished with a variety of agents ranging from simple water to specialized enzymes. To aid the process, degumming aids are also added to the bath. Furthermore, the efficiency of the process is influenced by the quality of the water, the pH of the bath, and the presence of particular salts.

2. Objective of Degumming of Silk:
Glycine, alanine, and serine are the primary components of fibroin, whereas sericin contains serine, aspartic acid, and glycine. As a result, any degumming procedure should focus on the peptide bonds that connect the amino acids that make up the majority of sericin. By removing sericin, as well as any impurities taken up during reeling and throwing, the primary objective of the silk degumming procedure is to give silk a soft handle and lustre. During degumming, some natural wax, colouring matter, and minerals may be removed. Sericin gum is found in about 20-25 percent of silk fiber. Sericin is a family of soluble glycoproteins produced by Bombyx mori’s middle silk gland. The silkworm’s silk is mostly made up of two proteins: sericin and fibroin; fibroin is the silk’s structural centre, while sericin is the gum which covers the threads and allows them to stick together. Even though sericin, which is also a protein, makes up the bulk of the gum or outer protective coat in a silk filament, the process primarily involves cleavage of sericin peptide bonds, either by hydrolytic or other methods, and subsequent removal of sericin from fibroin by solubilization or dispersion in water.

Objective of degumming of silk

3. Different Methods of Silk Degumming:

3.1 Extraction with water
At high temperatures, sericin can be removed from water. It is not a commercial process. By extracting sericin with water at 115°C for 3 hours, up to 96 percent of it can be extracted. Although it is still uncertain whether the removal is actually achieved through hydrolysis or dissolution.

3.2 Boil-off in soap
Silk degumming has been done for almost 200 years by boiling it off in soap solutions. Marseilles soap, which is made from olive oil, is the approved standard method. Many alternative soaps have been tried and shown to be effective. Palm-oil soap, lard-oil soap, and oleic acid soap are examples of these. The alkali produced by soap hydrolysis is responsible for the degumming action of soap solution. The alkali creates a chemical link with sericin, resulting in the formation of soda salt. The soap separates the swollen sericin, which then dissolves in water due to the soap’s emulsification effect. The amount of soap necessary for thorough degumming is determined by the type and nature of the silk. It’s an old method that appears to be still popular in the commercial world. The most popular soap is olive oil soap (also known as Marseille soap).

A typical recipe is as follows:

  • Marseille soap: 20-30 % owf
  • Time: 90-120 min
  • Temperature: 90-100°C
  • M: L: 1:40

The amount of sericin removed is also determined by the type of degumming soap employed. Soaps with the highest hydrolysis capacity are the most effective at degumming. Olive-oil soap is particularly ideal due to its high degree of hydrolysis, which results in a higher luster. There is less chance of excess degumming with soap because it has a moderate action and a high permeability. It also has a softening effect and improves the whiteness, lustre, and feel of the cloth.

However, there are certain drawbacks to using soaps to degum. The silk quality is influenced by the quality of the water used with the soap. Metallic ions in the water, like as calcium and magnesium, react with the soap to form an insoluble metallic soap that deposits on the fiber and dulls the lustre. Uneven coloring, yellowness, and embrittlement are all difficulties caused by residual soap in the silk. The amount of soap required is quite large (about 20–30 percent of the weight of the fiber) and, when discarded, pollutes the environment. Because Marseilles soap is expensive to import, degumming is frequently done with non-standard indigenous soaps based on sodium stearate.

3.3 Degumming with alkali buffer
Although alkali degumming is less expensive than soap degumming in terms of chemicals and labour, enhanced productivity, and so on, it might leave silk yellowish and abrasive. As a result, alkalis should not be utilised alone. The pH of the boiling bath should be kept between 9.5 and 10.5 when using alkalis. The pace of degumming is too sluggish below pH 9.5, while the risk of chemical damage is too severe above pH 10.5. Alkaline buffers such as sodium carbonate/sodium bicarbonate, sodium hydrogen phosphate/trisodium phosphate, and potassium tetraborate/boric acid have been utilised many times as a result. The most commonly used buffer is sodium carbonate/sodium bicarbonate. The concentration of electrolytes in the degumming bath, in addition to the buffer, influences the rate of degumming. The rate rises as the alkali content rises. Degumming takes 60 minutes at a pH of 10 and a temperature of 95°C with a 0.01 molar sodium carbonate/sodium bicarbonate concentration, but only 20 minutes with a 0.05 molar alkali concentration at the same pH and temperature.

The following is an example of a standard recipe:

  • Sod. Carbonate (0.5 M):5.3 gpl
  • Sod. Bicarbonate (0.05M):4.2 gpl
  • Non-ionic Surfactant: 3 gpl
  • Temperature: 96°C
  • Time: 20 min

The presence of salt in alkaline degumming fluid, particularly calcium salts, accelerates the degradation of silk. Sequestering compounds such as sodium hydrogen pyrophosphate have been shown to mitigate this harm. Systemic hydrolysis of peptide bonds causes degumming with alkalis, which is a non-specific process.

3.4 Enzymatic degumming of silk
Degumming of silk with enzymes is usually a two-step process. The cloth is treated with a solution containing soda ash for a particular period of time in the predegumming stage, and then further degumming is done in a solution comprising protease enzyme and non-ionic detergent in the second step. Since they may dissolve sericin without altering the silk fiber protein, several alkaline, acidic, and neutral proteases have been explored as degumming agents. Several proteases, particularly alkaline proteases, have been shown to remove sericin and improve silk surface qualities such as handling, shine, and smoothness. Trypsin, bacterial enzymes and papain are the most common enzymes used to degum silk.

Enzymatic degumming of silk

3.4.1 Enzymatic degumming has the following advantage over the conventional degumming with alkaline soap:

  • Because it has a specific reactivity, it may cause the least amount of damage to fibroin.
  • It is less likely to over-degum than alkaline soap degumming.
  • Weight loss can be easily altered by modifying the enzyme concentration, reaction time, and pH and temperature settings.
  • Silk is treated with enzymes at a low temperature (e.g., 40°C), which not only saves energy but also prevents fiber weakening.
  • Because enzymes biodegrade rapidly in nature, enzyme therapy is an environmentally beneficial technique.
  • The enzyme degumming procedure does not require the use of soap. As a result, the problem of uneven coloring caused by metallic soap can be avoided.

3.4.2 Enzymatic degumming also has some economic disadvantages as:

  • Because the gum must be swollen before the enzyme bath, some pre-treatment procedures are required.
  • It is a relatively slow reaction as compared to alkaline soap degumming. The silk is degumming in both hanks and fabric form.

3.5 Degumming with organic acids
Acids’ degumming effect appears to be related to the hydrolysis of proteins at certain amino acid sites. Acids target aspartic and glutamic acid residues, which are found in far greater abundance in sericin than in fibroin. Although acidic degumming has received little attention, it is considered to be safer than alkaline degumming. Effective degumming occurs with acids in the pH range of 1.5 to 2. Silk that has been degummed and tested with citric and tartaric acids has yielded positive results. Because mineral acids can cause significant damage to silk, only organic acids are often used.

The following is an example of a standard recipe:

  • Tartaric acid (0.05 M/L): 8 gpl
  • Non-ionic Surfactant: 3 gpl
  • Temperature: 110oC
  • Time: 60 min

It has been discovered that weight loss is almost completely independent of acid type for various acids. Dilute organic acids, unlike alkalis, target the peptide links that connect aspartic and glutamic acid. Attacks on the above link are 100 times faster than attacks on other bonds.

3.6 Degumming of silk using aliphatic amines
Degumming agents for silk have been found to be successful using aliphatic amines. Although methylamine produced the best results of all the amines studied, its low boiling point makes it unsuitable for industrial use. As a result, triethylamine is thought to be the most effective amine for degumming. Amines such as methyl, dimethyl, ethyl, and trimethyl can be employed. Because of its low boiling point, methyl amine is not appropriate for commercial application. Amines provide effective degumming (up to 90% sericin elimination) with minimal tensile strength loss. Amines can also remove impurities like processing oil etc.

A typical recipe is as follows:

  • Trimethyl Amine: 0.25 mol/L
  • N.I.D.: 3gpl
  • Temperature: 80oC
  • Time: 45 min
  • M:L ratio: 1:25

4. Comparison of Different Methods of Degumming of Silk:
On the basis of weight and strength loss after degumming, many approaches have been studied. The values with soap were used as the basis. It can be seen; soap, alkali, and acidic degumming remove the most gum. Alkali and acid, on the other hand, both induce significant strength loss, indicating silk damage. Amines, on the other hand, appear to be effective at both degumming and maintaining strength. While enzymes are good at keeping strength, they aren’t very good at removing sericin. This is a problem, particularly with textiles, because enzyme molecules are difficult to reach the interstices due to their huge molecular size. As a result, enzymatic degumming may be preferable for yarns over textiles. Degumming using amine appears to be the best option when all of the qualities are considered, followed by alkaline degumming.

5. Reuse of Sericin:
The degumming waste liquor, which is high in sericin, is used as a raw material for sericin powder manufacture. The sericin powder is used as a moisturiser, in hair-care products, and as a natural textile finish in the cosmetic business. The effluent is also significantly reduced when sericin is removed from the waste degumming liquor.

Reuse of sericin

6. Conclusion:
Degumming of silk is essential to obtain an ideal fiber because of its modified fiber structure by scouring the sericin and some impurities from silk fiber.

7. References:

  1. The dyeing of silk by K. Murugesh Babu, in Silk (Second Edition), 2019
  2. Textile chemical processing by NPTEL
  3. Studies in a degumming of silk with aliphatic amines by M.L. Gulrajani and Smita Sinha
  4. Comparative evaluation of the various methods of degumming silk by Sheetal Chopra and M. L. Gulrajani
  5. https://www.textiletoday.com.bd/silk-and-its-degumming-process/

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