Textile Bleaching [A to Z]

Last updated on October 7th, 2023 at 12:39 am

Definition of Textile Bleaching

Bleaching is the 3^rd step of wet processing. The process by which the natural color of a fibre can be removed and make the textile material pure white and bright is called bleaching.

Objects of Bleaching

There are many objects as follows:

• To remove the natural color.
• To make the fabric permanent white and bright.
• To increase the absorbency power of the cloth.
• To increase dye affinity.
• To ensure level dyeing property.
• To make the fabric suitable for the next subsequent process.

Types of Textile Bleaching Agent

They are in two types as follows:

• Oxidizing and
• Reducing

Oxidizing Bleaching Agent

The chemical agents which remove coloring material from textile materials by oxidizing reaction and make the material pure white and bright and to increase absorbency power is called oxidizing bleaching agent. Ex: O₃, Ca(OCl)Cl, H₂O₂, NaOCl, Ca(OCl)₂, K₂Cr₂O₇ etc.

Generally natural vegetable fibres are bleached by these bleaching agents.

Reducing Bleaching Agent

The chemicals which removes coloring materials from textile materials by reduction reaction and make the material pure white and bright and increase absorbency power is called reducing bleaching agent. Ex: H₂S, FeSO₄, NaHSO₄, Na₂SO₄ etc.

Generally protein fibre i.e. silk, wool, etc. are bleached by these agents.

Factors/Conditions of Bleaching Process

Bleaching process depends on some important conditions. Some important conditions are given below:

• Concentration of the chemical.
• M : L ratio.
• P^H of the liquor.
• Temperature of the liquor.
• Time of treatment.
• Catalyst.

Machine Chemicking Bleaching Process

Machine chemicking process is one of the discontinuous or, batching processes. This process takes more time.

Recipe:

HOCl: 3 – 4 Tw⁰

Temperature: Below 40⁰C

Time: 8 – 12 hrs

P^H: 9.2 – 11

Bleaching solution is to be prepared according to the above recipe. Here, material and liquor ratio is less. First the fabric is openly passed through the guide roller and immersed into the bleaching solution by immersion roller.

Then the fabric is squeezed by a pair of squeezing roller and pilling in a definite box. This fabric is kept in this box for a certain period for bleaching action. Then the fabric is treated with 0.5% HCl or H₂SO₄.

Faults in Bleaching and Their Remedies

Several faults of Bleaching are as follows:

• Incomplete and uneven bleaching
• Over bleaching
• Weaken the fabric.
• Roughness of the textile material.
• Yellowish effect over the textile material.

Incomplete and Uneven Bleaching

Causes:

• For using insufficient bleaching chemicals.
• Uncontrolled temperature.

Remedies:

• Sufficient chemicals should be used.
• Temperature should be controlled.

Over Bleaching

Causes:        

• For using excess/over chemicals.

Remedies:        

• Sufficient chemicals should be used according to the weight of the textile material.

Weaken the Fabric

Causes:        

• Prolong stay in the sunlight causes weaken the fabric.
• Incorrect concentration of the solution.

Remedies:        

• We should protect the bleaching process from the sunlight.
• Concentration of the solution should be correct.

Roughness of the Textile Materials

When we use calcium hypochlorite for bleaching purpose, hypochlorite is reacted with the atmospheric carbon-dioxide and thus produces calcium carbonate which is precipitated. Calcium carbonate is insoluble in water and if attached to the fabric surface that makes fabric rough.

Ca(OCl)Cl + CO₂ → CaCO₃↓ + Cl₂

Remedy:

• Fabric should be scoured for removing calcium carbonate.

Points to be Considered Before Bleaching

The points that are considered are given below:

• In case of proteinous fibre reducing type of bleaching agent is used.
• If proper bleaching is not done, it will not give proper result.
• Jute, Hemp, Flax type fibres required two types of bleaching chemicals, Firstly, oxidative is used and finally the reducing type agent is used.
• Most of the bleachings are performed in the presence of ideal bleaching agent. H₂O₂ may be termed as ideal bleaching agent because, it is very cheap. It has no side effect.

Bleaching of Cotton by Hydrogen Per-Oxide (H₂O₂)

Thenard produced H₂O₂ from BaO₂ and dilute HCl in 1818. He named this as oxygenated water.

BaO₂ + 2HCl → BaCl₂ + H₂O₂

Recipe:

H₂O₂ (35% S): 0.75 litre/kg (According to the wt. of the material)

NaOH: 0.05 kg

Na₂CO₃: 0.02 kg

Wetting agent: 1%

NaSiO₂: 0.07 kg

H₂O₂ (50%): 0.5 l/kg

H₂O: 100 l/kg

H₂O₂ is an oxidizing agent which is used largely for cellulose fibre bleaching. H₂O₂ is also used for protein fibre such as wool, silk-bleaching etc. In H₂O₂ bleaching, H₂O₂ releases hydrogen ion (H⁺) and per hydroxyl ion (HO₂^–). Here, P^H level is maintained 10.7 – 10.9.

Per hydroxyl ion is released for alkalinity of hydrogen per-oxide solution. This per ion bleached the textile materials. H⁺ ion has no bleaching action. It maintains P^H level.

(HO₂^–) + Colored materials = Bleached materials + H⁺

H₂O₂ is normally unstable, moreover Cu, Ni, Fe, etc. catalyst decomposes H₂O₂ and produces H₂O and O₂.

2H₂O₂ + Catalyst → 2H₂O + O₂

The produced O₂ has no bleaching action. So, bleaching ability can be reduced if water contain Cu, Ni, Fe, etc ion, then H₂O₂ catalytically decomposes and reduces bleaching ability. So, all the above matters should be considered in H₂O₂ bleaching.

You may also like: Objects of Singeing | Gas Flame Singeing Machine