Textile Bleaching

Textile Bleaching Agents
Bleaching Hypochlorites
Bleaching with Peroxy compounds
Bleaching With Sodium Chlorite
Reductive Bleaching Agents
Other Bleaching Agents
Bleaching
Gentle Bleaching
Textile Bleaching of Different Substrate
Cotton
Wool
Silk
Acrylic
Polyamides
Polyesters
Viscose Rayon
Jute
Linen
Blended fibres

Bleaching With Sodium Chlorite

Sodium Chlorite
Sodium chlorite, anhydrous is in the form of slightly hygroscopic crystals or flakes , soluble in water.The hydrated material is in the triclinic leaflets. The chemical formula is NaClO2 , and molecular weight is 90.45.  The molecular weight of hydrated product is 144.5.

Bleaching with sodium chlorite.(NaClO₂ )

The sodium chlorite is available as a powder , and it is applied under strongly acidic conditions to textiles. Its application produce a toxic and corrosive gas.

a. Bleaching Mechanism

Sodium Chlorite is an oxidant particularly adapt for synthetic fibre bleaching (polyamidic, acrylic, polyester) and cellulosic (man-made and natural, particularly for linen). Its oxidant action works thanks to Chlorine Dioxide which develops through an acidification process: therefore it is highly recommended to work with closed equipment and to furnish the bleaching departments with aspiration systems.

A general guideline for  bleaching bath at M:L=1:10 is 

b.  pH :- 

The pH of around 4 ± 0.2 required for bleaching is maintained with buffers or as termed in industry activators, like sodium acetate or sodium dihydrogen phosphate (NaH ₂PO₄) Latter is usually preferred because it improves whiteness o goods. Neutral or slightly acid chemicals that liberate acid on heating are also used occasionally. Organic esters like ethyl lactate or titrate and their ammonium salts are also suitable for this purpose

however the pH range for different substrates is as given below,

Polyamide= 3.5-3.8

Acrylic= 3.0-3.5

Cotton= 3.6-4.0

Linen=3.6-4.0

cotton/polyester=3.6-4.0

c.Temperature/Time

Polyamide=85-900C/90-120 min

Acrylic=90-95oC/60-90 min

Cotton/Linen/cotton-polyester=950C/45-60 min

d.Quantity of sodium chlorite

Polyamide =1-1.g gpl

Acrylic = 1.5-2.5 gpl

Cotton=0.3-0.5 gpl

Linen=0.3-0.5 gpl

Cotton-polyester=0.3-0.5 gpl

Stabilizers that control chlorine dioxide emission, such as borax or polyphosphates should be used.

e. Effect on metals

Since chlorine dioxide is highly corrosive in nature therefore it is recommended to use it in fully closed system ,and preferably in fully flooded machines . The gas is also corrosive and result into rusting of stainless steel surface, which stain the material to be bleached.A corrosion inhibitor such as sodium nitrate is essential in an quantity equal to the chlorite that moderates decomposition of the chlorite and inhibits corrosion of metals.

f.Water Quality

The water hardness and presence of metal ions does not affect the sodium chlorite bleaching under acidic conditions.

g.Antichlor treatment

Some cationic dyes are very sensitive to traces of chlorine and will rapidly fade giving poor colour yields, particularly when dyeing pale shades. An anti-chlor treatment of fabric bleached with sodium chlorite may be necessary and small additions of sodium bisulphate or thiosulphate to the dyebath will avoid problems with cationic dyes

h.Application process of chlorite bleach on cellulosics and synthetics

i.Advantage of sodium chlorite bleaching

          The principal advantages of Sodium Chlorite/Chlorine Dioxide for this application may                   be summarised as follows:

• high brightness degree (especially for acrylic fibres)
• negligible degradation of fibres (1-2% weight loss for cellulosic fibres and no attack to the polymeric chains in the synthetic fibres)
• lower environmental impact of wastewaters (negligible level of AOX and COD).
• Versatile bleaching agent for cellulosics , synthetics and blends ,especially poly-cotton blends. It is also safe for those synthetic fibers which are sensitive to alkali.
• Non sensitive to metal ions such as iron and Water hardness under acidic conditions.

      i. Disadvantages of Sodium Chlorite bleaching

• It is expensive as compared to hydrogen peroxide.
• It can not be used for wool and silk.
• Chlorine dioxide is highly corrosive to metals and toxic in nature.
• Bleaching in neutral and acidic pH damage cotton. 

Bleaching With Peroxy Compounds

Of the peroxy type bleaches, sodium perborate (NaBO₃4H₂O) is the least expensive. Organic peroxides such as dibenzoyl peroxide, are used in the bleaching of flour.Hydrogen peroxide is generally made from nonrenewable resources, but decomposes into hydrogen and oxygen

Bleaching with peroxides.

Hydrogen peroxide

 (H₂O₂), a colourless liquid usually produced as aqueous solutions of various strengths, used principally for bleaching cotton and other textiles.Hydrogen peroxide is a simple chemical compound. It's water with an extra atom of oxygen attached to it, H2O2. Hydrogen peroxide is valuable as an oxidizing agent (like ozone, or bleach for example) because it can release that single oxygen atom in the presence of another reactive substance. This reaction is called oxidation or bleaching.

Bleaching Process with Hydrogen Peroxide

Hydrogen peroxide bleaching can be done by

1.Batch wise, 

2.Continuous 

3.Semi continuous method.

a.Quantity of peroxide required in Bleaching

The quantity of hydrogen peroxide is used at 4- 6% on the weight of goods , but must be optimized to get the desired ground whiteness of the fabric.

b.Temperature

Cotton and Bast fibers are bleached at 80 - 95°C in bath processes, while blends of cotton and regenerated cellulose fibers are bleached at 75 - 80°C. The bleaching time is generally between 2 and 5 hours. In a pressurized high temperature (HT) apparatus cotton can also be bleached at temperatures of 110 - 130°C in only 1 to 2 hours.

c.Time

During the impregnation processes the temperature and as well the retention time varies widely. During a cold bleach process a dwell time of 18 to 24 hours is necessary. In the pad steam process under atmospheric pressure the bleaching time is generally between 1 to 3 hours. The above mentioned processes describe batch processes. Today a lot of continuously, intelligent finishing equipment exists in which the bleaching step is only one of some other treatments and the reaction time of the impregnated material in such steamer is only between 7 to 20 minutes. In general these bleaching process correspond to a preliminary bleach.

d.pH:-The pH value depend on the fibres to be bleached and pre-treatment.

NaOH is used in case of H2O2 bleaching. This is used to bring the PH upto 9-10 because H2O2 become active at this pH or oxidation is start at this pH.

For the bast fibres, such as linen, weaker alkaline or soda alkaline baths are used in order to avoid a cottonizing. Regenerated cellulose fibres are more sensitive. Therefore, they are only bleached in weak alkaline baths.
Alkali sensitive animal fibers must be bleached in very weak alkaline solutions. Phosphates and ammonia are most widely used as alkalization source. With tetrasodium pyrophosphate simultaneously a stabilization of the bleaching liquor can be attained.

e.Water Quality

Soft Water free of iron and copper impurities is recommended for peroxide bleach treatment.

f.Peroxide Stabilizers

High pH and temperature lead to the faster decomposition of peroxide bleaching liquor and degradation of cellulose.The role of the stabilizer is simply to control or regulate these effects the
act as buffers, sequestrates and in special cases, enhancing performance of the surfactant used in the bleach bath.

 For caustic alkaline bleach sodium silicate, organic stabilizers or the combination of both are suitable. In weak alkaline baths the addition of tetrasodium pyrophosphates can be used alone or together with an organic stabiliser.

Advantages of Peroxide Bleaching:
1.Among the oxidizing bleaching agents, only hydrogen peroxide provides a high bleaching effect     at reasonable costs, especially if modern short-term bleaching processes are used with only a        few minutes bleaching time.
2.Peroxide bleaching keeps the fibre quality intact.
3.Cotton can be bleached with peroxide in a single stage. Other processes require two or three
   bleaching stages,(desize with scour, scour with bleach and desize with scour and bleach).
4.No separate pre treatment is necessary because hot, alkaline bleaching has not only a bleaching but also a cleaning effect, it therefore combines  the advantages of an alkaline extraction with the bleaching treatment.
5.Animal fibres can only be bleached with peroxide to a high and stable degree of whiteness.
- Corrosion of stainless steel equipment does not occur during peroxide bleaching.
6.The spent peroxide baths still contain residuals of hydrogen peroxide which fever the                  degradation of the organic impurities in the effluent, and this helps to decrease the chemical        oxygen demand (COD).

Bleaching with sodium perborate

Sodium Perborate monohydrate (Synonyms: sodium peroxyborate, sodium peroxoborate, PB1) is similar to sodium percarbonate as a famous oxygen bleach agent. It has been described as a stable, solid form of hydrogen peroxide allowing its introduction into the wash at the same time as the detergent. Sodium perborate is a gentler bleach than sodium hypochlorite, causing less damage to fabrics and dyes, but by itself is only effective at high (>60ºC) temperatures. Although solid chlorine bleaches exist, they are rarely used in laundry detergents.

 The oxidative power of sodium perborate improves the cleaning, bleaching, stain removal and deodorizing performance of powder detergent formulations, all fabric dry bleaches, denture cleaners, automatic dishwasher detergents and various institutional and industrial laundry products.

Bleaching with peracetic acid.(CH3COOOH)

Peracetic acid is a colorless liquid with a pungent, sour smell and is included in the chemical group of hydroperoxides. It is highly oxidant and corrosive and can explode at temperatures exceeding 110ºC. It is more fat-soluble than hydrogen peroxide.

Peracetic acid is produced by the chemical reaction of acetic acid and hydrogen peroxide. It works in a very narrow pH range of 7 to 8. Below pH 7.0 the bleaching is not proper and above pH 9.0 fiber degradation takes place. Peracetic acid is used a bleaching agent for nylon, and acetate where hydrogen peroxide can not be used. It can be used for viscose rayon and even cotton also.

Per-carbonates

Sodium percarbonate (Synonyms: sodium carbonate peroxyhydrate, sodium carbonate peroxide, PCS) is a free-flowing powder with a common name of solid hydrogen peroxide, it is an addition compound of sodium carbonate and hydrogen peroxide. Percarbonate has an active available oxygen content which is equivalent to 27.5% H2O2. It breaks down to oxygen, water and sodium carbonate upon decomposition. It's an environmentally safe bleaching agent.

Per-carbonates have increased in use as bleaching agents due to their ‘kind nature’. Per-carbonate behave quite similar to the per-borates by creating active oxygen and they also needs stabilisers for storage, but the active effects starts at 400C and hence no activator is needed. The reaction products – carbonates – are substances naturally found in the environment.

Percarbonate is used as an activated oxygen component in detergents, bleaches and cleaning agents.

Persulfates

Ammonium and sodium persulfates are used in the desizing and bleaching of textiles and the development of dyestuffs.

Molecular Formula : Na2S2O8 Molecular weight : 238.13
	Sodium persulfate is a white,crystalline,odourless salt.It is used as initiator for the Polymerization of monomers and as a strong oxidizing agent in many applications.It has the particular advantage of being almost non - hygroscopic,of having a particularly good storage stability as a result of its extremely high purity and of being easy and safe to handle.