Scouring and Bleaching in Textiles

In this article, we are going to know about the scouring and bleaching process of cotton fabric. We will know about the recipe and chemicals required for these processes. Let's start with the introduction.

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Introduction

Scouring is a process where the natural impurities of cellulose fibers are normally removed (scoured) by boiling off in a strongly alkaline solution. Natural impurities such as mineral matter, oils, waxes, and ashes hinder the penetration of dyes and chemicals into the interior of fibers; resulting in a huge wastage of dyes and chemicals and improper scouring creates uneven dyeing. Alkali treatment with auxiliaries at high temperatures removes the inherent and added impurities from cotton and other cellulose fibers. Treatments are similar, but the concentration of alkalis and process parameters should be adjustable according to composing amount and structure of impurities.

Bleaching is a process by which the natural colors of textile fibers are removed to achieve whiteness for the development of true color during coloration. It's an oxidation process whereby coloring matters are destroyed; it can't remove dirt or even coloring matters. Although, in the dark shade, this process is not compulsory. But for the pale and brilliant shade it is absolutely mandatory, and for medium shade modified light bleach is sufficient. This process can be done in several ways – bio bleach or chemical bleaching. The chemical is still very popular due to its good bleaching results. Strong and uncontrolled bleaching action can degrade the cotton fiber.

Recipe

Chemicals/Parameters	Quantity
NaOH	3g/L
H2O2	4g/L
Stabilizer	1g/L
Detergent	2g/L
Wetting agent	1g/L
Sequestering agent	1g/L
pH	10-12
M:L	1:40
Time	30–40 minutes
Temperature	98-100℃

Calculation

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

Fabric = 5 gm

So, liquor = 5 ✕ 40 = 200 ml

M:L = 5: 200

Chemical = (g/L ✕ Total liquor) / (1000 ✕ Stock solution %)

Examples:

For 5% stock solution of NaOH = (3 ✕ 200) / (1000 ✕ 5%) = 12 ml

For 5% stock solution of hydrogen peroxide = (4 ✕ 200) / (1000 ✕ 5%)= 16 ml

Similarly, calculate amounts of other required chemicals.

Then sum up the total required chemicals.

Then required freshwater = (200 – Total required chemicals) ml.

Scoured and Bleached Fabric

Scoured and Bleached Fabric

Process Curve of Scouring and Bleaching

Scouring and Bleaching Curve

Auxiliary Chemicals Function

Caustic soda: Neutralizes acidic materials, saponify glycerides (waxes and oils), and solubilizes silicate.

Detergent: For emulsifying fats, oils, and waxes.

Stabilizer: Stabilizer (usually sodium silicate) is used to stabilize the bleaching bath. Water hardness has a positive impact on the stability of the bleaching bath; when the stabilizer is used, fluctuations in the hardness are of no importance, and it controls hydrogen peroxide.

Hydrogen peroxide: It gives a very high and stable degree of whiteness.

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.

Sequestering agent: In the absence of a water softening plant in an industry, it is also possible to reduce the unwanted interference of heavy metal ions during soaping or dyeing by a sequestering agent. A sequestering agent is a substance that suppresses a property or reaction of metal ions without the removal of that metal from the system. The most common sequestering agent is EDTA, known as ethylenediaminetetraacetic acid.

Conclusion

The bleaching produces a high degree of whiteness and complete removal of seed husks from a fabric. But this process must be regulated carefully. On the other hand, scouring removes natural impurities including oil and waxes the goods become more absorbent. So, the fiber gains more absorbency for dyeing.