Dyeing of Silk With Acid Dye

Theory

Silk is an animal fiber, and it is also called protein fiber. Silk is the only natural fiber that is found in filament form. Silk is produced by insects. Fibroin is the main chemical component of silk. In raw silk, 75% is Fibroin (insoluble protein) and 25% is Sericin called silk gum. Because of being a protein fiber, it has 20 types of amino acids and carbonyl groups present. For this, silk shows cationic character. So it can be dyed with an anionic dye such as acid dye. The acid dyestuffs are so-called because firstly the original members of the class were applied in a bath containing mineral or organic acid and secondly because they were nearly all sodium salts of organic acid and anion is an active color component. Acid dyes are usually sodium salts of sulphonic acids, or less frequently carboxylic acids For example, acetic acid is, therefore, anionic in an aqueous solution. They will dye fibers with cationic sites. These are typically substituted by ammonium ions like silk. This fiber absorbs acid. The acid protonates the fiber's amino groups, so it becomes cationic. Dyeing involves the exchange of the anion associated with an ammonium ion in the fibers with a dye in the bath.

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Objectives

To know about the chemical properties of silk
To know about acid dye
To know about the dyeing of silk fabric
To know about chemical auxiliaries.

Recipe

Chemicals / Parameters	Quantity
Acid dye	1.5%
Wetting agent	1 g/L
Leveling agent	2 g/L
Acetic acid	1 g/L
pH	4.5-5.5
M:L	1:40
Time	20 mins
Temperature	95℃

Calculation

M:L (Material: Liquor) = 1:40 (M:L is more because it is done in the open bath)

Fabric: 2 gm

So, liquor = 2 ✕ 40 = 80 ml

So, M: L= 2:80

We know, Dye = (Sample weight ✕ Shade%) / (Stock solution%)

For 1.5% shade and 1% stock solution,

= (2 ✕ 1.5%) / (1%) = 3 ml

Again, Chemical : (g/L ✕ Total liquor) / (1000 ✕ Stock solution%)

For 5% stock solution wetting agent required = (1 ✕ 80) / (1000 ✕ 5%) = 1.6 ml

For 5% stock solution levelling agent required = (2 ✕ 80) / (1000 ✕ 5%) = 3.2 ml

Thus, calculated total chemicals.

Now, freshwater needed = (80 – Total chemicals) ml

Sample of Acid Dyed Silk

Dyeing Curve

Auxiliary Chemicals Functions

Wetting agent: It accelerates the wettability of material in solution, thus helping the easy penetration of chemicals into the substrate. It reduces the surface tension of water to allow it to spread drops on a surface, increasing the spreading abilities of a liquid.

Acetic acid: Its function is to control pH in the process.

Acid dye: Acid dye is a dye that is typically applied to textiles at low pH. They are used to dye wool, silk, and nylon. Generally used to dye protein fibers. It is highly water-soluble and gives good lightfastness. It contains sodium salt or sulphonic acid, so acid dyes are water-soluble. In water and acetic acid, the NH2 functionalities of the fibers are protonated to give a positive charge: NH3+, so that there's an interaction with the negative dye charge, allowing the formation of ionic interactions. Also, Vander Walls bonds, dipolar bonds, and hydrogen bonds are formed between dye and fiber.

Leveling agent: Leveling agent tends to slow down the dye uptake of the fibers, helping to produce a more uniform color in the textile fiber. They are also termed retarding agents or retarders.

Conclusion

The acid dyes are used to dye protein fibers such as silk, wool, angora, alpaca, mohair, feathers, etc., and even man-made nylon which is chemically similar to silk. In this experiment, we knew about the dyeing process of silk fabric and the functions of different chemicals.